Part 1 Background: When one is immersed in a subject, the way we are here at megapixel.net, it is easy to lose sight of the fact that the technology , and its jargon, can be a bit intimidating to someone approaching the subject for the first time. This little guide is an attempt at at some demystification, and will endeavour to show a path to a better understanding of digital photography , using some of the resources available here, and in other valuable sites, on the Internet. Until very recently , a basic amount of computer literacy was needed to seriously consider a digital camera as a replacement for a film one. This is changing fast. Many stores that used to only offer film developing and printing, are now offering their customers the possibility of bringing in their digital images for printing, much like they had done with their rolls of film. The difference is that these images are not recorded on film, but as digital information stored in one type of "memory" or another. And, that once the images have been copied, the customer gets the "digital film" back, to reuse, again and again. This development has opened the door of digital photography to a much wider audience. No longer is it an absolute requirement to have, and know how to use a computer . Many are realizing that digital photos do not suffer from film's shortcomings: fading colours, loss of the negatives, etc. This, combined with lower digital camera prices, has increased the attraction of d igital photography . A look at the basics: Digital cameras may almost appear magical, but they're not. The technology employed to capture an image without the use of film is not really new, only recently more affordable. Instead of film, the image is formed b y the camera's lens onto an image sensor , broadly similar to those used in video cameras. The light is gathered by tiny elements on the sensor which are referred to as "pixels". Each of these tiny sensors detects the amount of light falling on them, as it is filtered by an overlaid colour mask. In this way, the light gathered at the location of any given pixel can be attributed to either red, green or blue, the basic colour components of the photo. This colour information is then "processed" by the electronics in the camera so that the colour values gathered from all the locations on the sensor are organized precisely, creating a "map" indicating clearly the physical location of all the colours and their intensity. The result is a digital image. This electronic information is then recorded, in a digital file, each bit of information processed by the camera being encoded as a "1" or "0" value, sequentially , which can later be read by another digital device, such as a computer, or a printer. It is this digitization process which offers the true value of digital photography . Indeed, once the image is in this digital format, it is nearly indestructible, and can be copied time and time again without ever losing its integrity. The differences between a film camera and a digital camera The differences between film and digital cameras can be placed into two categories: the camera, and the system it uses to store the images. Let's start with the camera. The camera: Unlike a film camera, the digital camera is not only the tool to focus the image, but also the device that records it. In a way, a digital camera is both camera and film. In a sense, when a digital
When one is immersed in a subject, the way we are here at megapixel.net, it is easy to lose sightof the fact that the technology, and its jargon, can be a bit intimidating to someone approaching thesubject for the first time. This little guide is an attempt at at some demystification, and willendeavour to show a path to a better understanding of digital photography, using some of theresources available here, and in other valuable sites, on the Internet.
Until very recently, a basic amount of computer literacy was needed to seriously consider a digitalcamera as a replacement for a film one. This is changing fast. Many stores that used to only offerfilm developing and printing, are now offering their customers the possibility of bringing in theirdigital images for printing, much like they had done with their rolls of film. The difference is thatthese images are not recorded on film, but as digital information stored in one type of "memory" oranother. And, that once the images have been copied, the customer gets the "digital film" back, toreuse, again and again.
This development has opened the door of digital photography to a much wider audience. No longeris it an absolute requirement to have, and know how to use a computer. Many are realizing thatdigital photos do not suffer from film's shortcomings: fading colours, loss of the negatives, etc. This,combined with lower digital camera prices, has increased the attraction of digital photography.
A look at the basics:
Digital cameras may almost appear magical, but they're not. The technology employed to capturean image without the use of film is not really new, only recently more affordable. Instead of film,
the image is formed by the camera's lens onto an image sensor, broadly similar to those used invideo cameras. The light is gathered by tiny elements on the sensor which are referred to as"pixels". Each of these tiny sensors detects the amount of light falling on them, as it is filtered by anoverlaid colour mask. In this way, the light gathered at the location of any given pixel can beattributed to either red, green or blue, the basic colour components of the photo.
This colour information is then "processed" by the electronics in the camera so that the colourvalues gathered from all the locations on the sensor are organized precisely, creating a "map"indicating clearly the physical location of all the colours and their intensity. The result is a digitalimage.
This electronic information is then recorded, in a digital file, each bit of information processed by thecamera being encoded as a "1" or "0" value, sequentially, which can later be read by another digitaldevice, such as a computer, or a printer.
It is this digitization process which offers the true value of digital photography. Indeed, once theimage is in this digital format, it is nearly indestructible, and can be copied time and time againwithout ever losing its integrity.
The differences between a film camera and a digital camera
The differences between film and digital cameras can be placed into two categories: the camera,and the system it uses to store the images. Let's start with the camera.
The camera:
Unlike a film camera, the digital camera is not only the tool to focus the image, but also the devicethat records it. In a way, a digital camera is both camera and film. In a sense, when a digital
camera is selected, so is the film: the sensor that records the image.
Since the image is captured by the pixels of the sensor, the quantity of pixels will determine theoverall quality of the image. In general this value is referred to as the resolution and is commonlyexpressed as the total number of pixels (1, 2, 3 megapixel, "mega" meaning million); or the numberof pixels used to make up the image, stated as horizontal and vertical values, as in: 2048 x 1536pixels.
A corollary to the resolution of the sensor is that it also places a limit on the size of the images atthe time of printing, be it when the image is printed in a store, or printed on a personal colourprinter. Indeed, to create a smooth image, one that can be compared to a print made from film, acertain density of pixels per square inch is required, and this minimum density, in turn, dictates thedimensions of the print.
Image storage:
As mentioned above, the other important thing to consider with a digital camera is the way itsimages are stored. The digital images are generally stored using a type of flash memory, which is amemory that does not require a constant source of power to retain the information stored in it.
Recently, another way to store digital images was introduced, the CD. However, unlike the flashmemory used generally, the images are recorded once, and the disks cannot (so far) be reused.
Leaving the CD-ROM type of storage aside, the flash memory designed for digital cameras (knownas memory cards) come in different flavours, each more or less popular, and each offering varyingprice and capacity advantages and disadvantages. The two most wide spread formats currently are:CompactFlash and SmartMedia. To date, CompactFlash memory cards can be purchased in largercapacities than can SmartMedia cards, but they are generally more expensive.
Part 2
What's available:Digital cameras can be grouped into five broad price ranges. Keep in mind thoughthat these groupings are rough (the price ranges given are in US Dollars):
Inexpensive: below $150
Entry-level: around $200 to $500
Mid-range: around $600 to $900
High-end: around $1,000 and up to $2,000
Pro: $2,000 and up.
For our purposes here, "inexpensive" cameras would be the ones that use small
lower-priced CMOS sensors, much like those found in webcams. The cameras willcapture a photo, but will usually not have an LCD screen to display it, or photoquality optics. They can be used for fun and as introduction to digital photography,but little else. Generally, image resolution is 640 x 480 and below, and some mayuse interpolation to reach their highest resolution.
Entry-level cameras are usually much more versatile. They may not have a zoom(although some do), but will usually have an autofocus lens, and are generallydesigned to be easy to operate. The large majority use CCDs as opposed to smallCMOS sensors. Their resolution is from 1 or 1.3 megapixel, and most of thesecameras are able to produce excellent photos which can be printed up to a 6 x 4inch size.
Midrange is a very broad category. It includes products by very well-known namesin photography, and models that are generally in the 2 to 3 megapixel range.These are cameras that can produce images which can be printed at least to an 8 x6 inches size or more. They also offer more advanced features such as priority
modes, 3 to 5X zoom lenses, and features such MPEG (mini-movies) capability. Theyare usually designed along the lines of compact cameras, but can also offer a fewfeatures normally reserved for high-end cameras. Usually, these models will have anoptical viewfinder, but will not offer the more complex TTL viewfinders.
High-end cameras emcompass cameras that offer extensive controls and expensive
optics such as stabilized long zooms. These cameras generally use high resolutionCCDs that are 2 or 3 megapixel and above, and normally have varied shootingmodes, very precise optics and 2 or 3 different metering systems. Some of thesemodels may offer TTL viewfinders of one type or another, compatibility with morethan one type recording media (CompactFlash and SmartMedia), compatibility withexternal flash units or their own dedicated units, remote controls, etc.
Pro covers the upper prince range of digital cameras, from Digital SLR bodies todigital camera backs. Price wise, the sky is the limit, but there are a few modelswhich are excellent and more affordable than others: Nikon D1, FujiFilm FinePix S1,and the Canon D-30.
Again, these are very broad categories. In fact many camera models easily overlapfrom one group to the next. Features such as a powerful zoom, image stabilizersetc. will make one camera more expensive than another, even though they mayboth have a similar image resolution.
While a host of features are nice, the most critical element has to be the imagequality of a camera, after all, what good is very high resolution and fancy features if the image quality is so-so? The image quality of any given camera is also the mostdifficult to gauge and is not guaranteed that the camera with the highest resolutionwill have the best image. High resolution won't help a camera that has alackadaisical white balance, or an imprecise metering system. Digital cameras arevery complex and many critical elements must function well for the camera toproduce a high-quality image.
Many opinions are available to help the buyer, from reviews such as the ones foundhere and other Internet resources (see the links page), to magazines and useropinions. However, when reading reviews keep in mind that some cameras willproduce extremely good photos in places where the sun shines copiously, but yieldfairly grainy images in places where the sun is not as strong, or the light as bright.Therefore, it is important to try to find more than one opinion and to take note of where (geographically) the review originates. The opinion of someone that has usedthe camera under generally similar conditions to yours could prove valuable.
Accessories:Card Reader:
A digital camera may not require the ongoing expense of film, but it will requiresome accessories nevertheless. One of these is the card reader. When it is attachedto the computer will make the camera much easier to use. While most camerasnowadays offer USB compatibility, and therefore fast image downloads, this stillrequires the camera to be plugged in, and often the addition of a DC adapter toavoid draining the batteries. All this tends to lessen the usefulness of the camera.For many, the fiddling with cables quickly becomes a pain and the camera isn't usedas readily. Card readers are relatively inexpensive ($30 to $80 US) and make thingsvery quick since all that is involved to see the photos is taking the memory card outof the camera, and inserting it in the reader.
A camera bag is another important accessory. Digital cameras are sensitive totemperature changes and humidity, more so than their film counterparts. A goodcamera bag will protect and insulate the camera from the elements. Particularly if the camera is left in a car for a period of time. Some camera bag manufacturers arenow making small bags specifically for digital cameras. The bags include storagepockets for memory cards, and for the next important accessory on our list:
batteries.
Batteries:
Some digital cameras come with their own rechargeable batteries, including somethat provide a long-lasting Lithium Ion battery. Most others though, take from 2 to 4AA sized batteries and will gobble up regular alkaline batteries at an alarming rate.A set of four such batteries can be appear to be drained in a matter of minutes,causing the camera to shut off.
In fact the likelihood is that the camera is not gettingsufficient power and that if the batteries are given a
rest, the camera can continue to operate. Still, a lessfrustrating alternative is use Nickel Metal Hydride(Ni-MH) batteries. These batteries last longer andcan be recharged easily, time and time again. Manybrands are available and their cost is quitereasonable. As for the cameras equipped with theirown battery? A fully charged spare is still a goodprecaution when the camera is going to be usedextensively.
Memory cards:
The ones that come with cameras generally lack sufficient capacity to be useful if the camera is used at its best image quality. Cards come in a wide variety of capacities and are easy to swap. Consider acquiring more memory cards, but don'tget stuck on the idea of having the most capacity available. It can be more practicalto have a few smaller capacity cards (32, 48MB) than one single high capacity card.
With the camera set to the best JPEG image quality,expect to need the following to be able to storeapproximately 36 pictures:
1.3 megapixel camera: 16MB
2.1 megapixel camera: 32MB
3.3 megapixel camera: 64MB
4 megapixel + camera: 96MB
In the next instalment, we'll look at a few questions to ask before deciding on acamera, and see how answering them can help direct you to the type of camera thatmay best suit your needs.
Part 3: Shopping for a camera.
When choosing a camera, there is no perfect way to proceed. However, it is possible to establishsome basic requirements which can help narrow the field. It is probably best to set a budget as afirst step. A budget with a latitude of a couple of hundred dollars might be easier.
The following questions & answers should provide some guidance:
Do you prefer a simple use point and shoot camera?
An answer in the affirmative to this question means that your primary consideration will be cost.From entry-level on up, all cameras offer a Program AE mode that is fully automatic. If on the other
hand you prefer a camera with more options, then you are able to eliminate most entry-levelcameras and direct your attention to the midrange.
Is a zoom critical for your needs?
A "yes" answer here, means you can focus your attention on models with 3X zooms and above.Ignore digital zooms altogether, these are simply cropping systems and the same results can beobtained from any image editing program. Concentrate on true optical zooms, and particularly oncameras that offer zooms that "bracket" a standard focal length of 50mm (in 35mm equivalent).Zooms that cover a range of 35mm to 105mm offer a reasonably wide angle and telephoto. Beaware that a zoom covering 28mm to 84mm, also 3X, will give you a wider field of view in wideangle, but considerably less magnification in telephoto.
If the camera will be primarily used in a work environment, then this should also be considered.Some work-related photography involves shooting indoors, where a wide angle is preferable, suchas a 28 to 84mm. Conversely, work-related photography done mostly outdoors may require a zoomwith a greater magnification, such as a 35 to 105mm. Most mid-range and High-end models offerthese type of focal lengths, and some go beyond 105mm. This leads to the next question:
What will be the primary use of the camera? Work or pleasure?
Often, a "work" camera will see greater use, and possibly by more people, than one intended strictlyfor personal use. This implies that the camera will sustain more wear and tear. In such a case, itmight be worthwhile to consider cameras that have metallic alloy surfaces. Usually, these kinds of
surfaces will wear better over time and use, than those that are made of plastics. Another possibilitymight be to look to weatherproof cameras; some models are available from Kodak and Fujifilm.
In addition, the camera's flash capabilities might be critical in work-related use. Most built-in flashsare insufficient to light a wide area. If a powerful flash is critical, then the camera should either havea hotshoe, or at least an external flash connection. These features are fairly common on midrangeand high-end cameras, but not on entry-level models.
What will be the primary application for your photos?
This is one of the most important question since, to a great extent, it helps determine the resolutionof the camera. Photos have many applications ranging from printing glossy 8 x 10's, to posting themon a Web site or e-mailing. Nearly all cameras on the market can accomplish the least demandinguses, but fewer will accomplish the most demanding.
Generally speaking, the Web is one of the least demanding uses for a camera. Because of theconstraints imposed by monitor resolution and bandwidth, photos destined to be posted "as-is"hardly ever require high resolution. But, if the photos are going to be altered or manipulated in aphoto editing program, then a higher starting resolution might be necessary even though theimages may end up smaller when used on the Web.
For instance, product photography for a Web site may necessitate that the images be presented ona uniformly coloured background. To ensure consistent results, this usually requires that theproducts be "cut-out" from the background of the photo and placed on another background texture
or colour. In such cases, a higher resolution makes the work easier since it provides greater detailand often produces a better image even when reduced in size.
If the photos are intended for printing—replacing the work done by the mini-lab—or a scanner, thena higher resolution might be necessary to produce sharper and larger prints. Photos with higher
resolution photos can be printed at a higher density and will produce a smoother image. For printingpurposes, the resolution is in fact the factor that limits the image size.
Too few dots of ink produce printed photos with a noticeable grain, or worse, jaggies. If a 300 DPI isassumed, a 1.3 megapixel resolution will produce a good quality 4 x 3 print; a 2 megapixelresolution will produce a 5 x 4 print; and 4 megapixel will produce a 7.5 x 5.5 inch photo. Note thatdepending on the printer, a lower DPI can be used to increase the image size without seriouslyaffecting the image quality.
The final step is to look at the answers to these basic questions, and line-up the candidates. Then,reading the reviews, available here and on other sites, for the "short list" of cameras, should helpnarrow the choices till your goal is reached.
This article is intended as a synopsis of desirable features to have on a digitalcamera, things that should be considered when selecting a camera.
We have purposefully left out some items for consideration: the ergonomic design of a camera; the type of viewfinder; the size and resolution of the LCD monitor; etc.Instead, we have tried to distill those things we judged to be the most importantelements.
Resolution:
As of this writing, and excluding special application cameras, digital cameras areavailable with sensors that cover a range from 640 x 480 (307,200 pixels) up to 14megapixel (14 million pixels). For most consumers though, the current economic
limit is at 8 megapixel.
A word of caution when considering resolution, the true resolution of the sensoris what matters, meaning the number of pixels physically present on the sensor, andnot the resolution of the image. If the image the camera produces is biggerthan the number of pixels on the sensor, for instance the CCD is 3 megapixel butthe image size is 6 megapixel, the image is interpolated.
Strictly speaking, the higher theresolution the greater the detail thecamera is able to capture, meaning thatan 8 megapixel image will always havemore detail than a 1 megapixel image.Likewise, larger resolution images resultin more detailed prints, and if desired,larger prints.
But, when the gap in resolution is not aslarge, such as is the case between 3.2megapixel and 4 megapixel, other
factors can tip the balance towards acamera with a lower resolution, and itsphotos may end up being more detailedthan those of the higher resolutionmodel.
One of the factors that can impact the amount of detail a camera captures iscompression.
(Related articles: Why resolution matters; Resolution vs Image sharpness; Interpolation; Howmany megapixel do you need?)
Compression and image formats:
One of most important things to look for when purchasing a camera is the type andvariety of image file formats it offers. The vast majority of images captured withdigital cameras are stored in JPEG format. JPEG uses a lossy compression algorithmthat reduces the colour hues of the image in order to reduce its file size therbylessening the time it takes to save it, and the space it takes up on the memory card.
The downside of the JPEG format is thatif the compression applied to the imageis too strong, the quality degradesvisibly, and images become much less
detailed than they should be. It istherefore critical to ensure that anycamera offer a good selection of compression settings.
A camera should offer at least 3 imagecompression settings, and one of theseshould offer a limited compression,creating files that are no less than 1/4to 1/6 the uncompressed file size.
If possible, an uncompressed format should also be available (TIFF) and/or a RAWformat that simply stores the image captured by the sensor "as-is".
(Related article: Compression)
Lens:
Most compact digital cameras have an optical zoom lens — not to be confused witha digital zoom (see "Related articles below) — and zoom lenses should meet somebasic minimums.
A lens should have a fairly bright maximum aperture — f2 or f2.8 — and should
correct for distortion: barrel distortion at the wide angle end, pincushion distortionat the telephoto end and chromatic aberration.
If a long telephoto lens is underconsideration, the maximum apertureavailable at the telephoto end should bechecked. Lenses that have relativelysmall apertures at the maximumtelephoto position may require a lot of ambient light to avoid camera shake.Alternatively, it may be advisable to lookto a camera that offers a stabilized lens.Additionally, very long telephoto lens
tend to produce a better image contrastwhen LD (Low Dispersion) glass is usedin their construction.
The shutter speed range of the camera should be checked. Cameras that have thecombination of bright lenses and high shutter speed are better at stopping fastaction. Similarly, cameras that are capable of long exposure times — at least 8
seconds — are able to capture photos at night. Ideally, the shutter speed range of the camera should extend to both fast and slow shutter speeds. Moreover, a cameraable to capture long exposures should offer a noise reduction feature.
(Related articles: The effect of shutter speed on the image; Night Photography; Noise: what it
is and when to expect it)
Shooting modes:
The availability of a good range of shooting modes is an important element of acamera. Users that want point and shoot simplicity should also look for cameras that
provide a variety of Scene modes. Scene modes automatically set a number of shooting parameters for specific types of image types. While not all these modes areuseful, a handful of basic scene modes can be very useful:
• Portrait • Night Scene • Landscape
• Sports • Beach/Snow • Sunset
For users that want the flexibility toexperiment with the way the cameracaptures images, the more "hands-on"Aperture Priority, Shutter Priority and
Manual modes should be available.Moreover, the ease of use of the modes— the way shutter speeds or aperturesare adjusted — should be checked.
Burst (also called Sequential) shootingmodes should also be part of thepackage. These modes are designed tocapture a series of images quickly so asto overcome the shutter lag of thecamera. While many cameras nowadaysexhibit very little shutter lag, autofocus
and metering still require a bit of time.Burst modes go a long to overcomingthis by allowing the photographer tocapture as the action unfolds.
(Related article: How to use continuous or burst modes)
Other important features:
Some other features are worthwhile having for everyone:
Adjustable ISO: means that the camera can be forced to a specific sensitivity. Thisprovides control over noise, and can be useful as a tool to expand or contract theshutter speeds available to the camera.
(Related article: The importance of an adjustable ISO; High ISO settings)
White Balance: just about every camera nowadays offers a control over whitebalance, that makes it possible to set the white point for a specific colour of light(cloudy, shade, tungsten, fluorescent, etc.) should the Auto mode not be able toproduce accurate colours. One setting however is not yet universally offered, a
Custom or User-set white balance that allows setting the white balance for aparticular light source that is not part of the camera's presets. Yet, that featureshould also be considered a necessity.
(Related articles: White Balance; Adjusting colour balance)
Exposure Compensation: exposure compensation serves to adjust or tweak howthe metering evaluates the subject. Most cameras offer compensation currently, andsome go further and even offer Flash compensation. A range of ± (plus or minus) 2EV is common, usually adjustable over 1/3 EV increments but sometimes in 1/2EVincrements. While exposure compensation may seem a bit daunting, it is in fact avery simple system that serves to shift where the metering assumes the "perfect"exposure point to be, by moving it slightly towards a brighter or darker image.Exposure compensation is useful in a variety of situations where the conditionsmight cause the metering to misjudge the light.
Exposure compensation, and if possible its automated version, Auto bracketing,should also be considered absolutely necessary.
When printing an image, the amount of detail the print will show is directly relatedto the resolution of the camera. Put simply, the higher the resolution, the better andsharper the printed image will be.
To illustrate this relationship between print sharpness and image resolution, asimple demonstration has been set up. A Minolta F300, a five megapixel camera, isused to capture 4 photos of the same subject and from exactly the same position,each at one of the camera's 4 available resolutions (2560, 2048, 1600 and 640).The photos are then printed on a Sony Digital Photo Printer (DPP-EX5) as borderless6 x 4 inch prints.
As with many digital cameras, the F300 records an
image with a 72 ppi (pixel per inch) resolution*. Thepixel per inch count of a digital image has arelationship to the DPI used in printing, as it relates tothe 1:1 image size that can be produced from a photowithout any up, or down, sampling.
In our example, the printer used requires 403 pixelsper inch, and images that cannot produce a 6 x 4.5inch print at 403 pixels per inch require some degreeof up-sampling to meet the needs of the printer.However, the greater the up-sampling is, the coarserthe image becomes.
* The ppi value of 72
originates with the ppiof colour monitors.Most were limited to 72pixels per linear inch.Nowadays mostmonitors have ppivalues of 96, and manydigital cameras recordimages with a higherppi than 72.
At this point, a look at what ppi represents, and how it relates to dpi is probably inorder. The pixels per inch (ppi) of a digital image is really a bit of an arbitrary valueas it does not assign precise dimensions to the pixels. The dpi, on the other hand, isa precise measure of the numbers of dots per inch used to print the image with a
With the ppi, one manufacturer may set it to 72 ppi — as we saw above, a leftoverfrom the resolution of older monitors — while another will peg it at 180 ppi, and yetanother at 300. To the user, the only real function of the value used for the ppi is toprovide an idea of the print size that can be produced by the resolution of any givenimage.
Let's take a look at an example, and see how variations in the ppi value of a 3-megapixel image affects the image size if we treat ppi as having a 1:1 relationshipwith dpi :
2048 x 1536 at 72 ppi = 28.44 x 21.33 inches or 72.25 x 54.19 cm
2048 x 1536 at 180 ppi = 11.37 x 8.53 inches or 28.9 x 21.67 cm
2048 x 1536 at 300 ppi = 6.82 x 5.12 inches or 17.34 x 13 cm
Since most printing devices — inkjet, laser and dye sublimation — output images atdpi values ranging from 150 to 400, the higher ppi numbers provide a better idea of what will be realistically possible in terms of print size. The values are however of
limited importance, in and of themselves.
With our test images, the chart below shows the differences between the resolutionof the image and the size it can produce at the resolution of the printer. Remember,these numbers are in relation to our output device.
ResolutionImage Size if the ppi is
set to 403 (printer res.)
PPI of the image
when the image sizeis forced to
6 x 4.5 inches(15.24 x 11.43 cm)
640 x 480 (VGA) 1.588 x 1.191 in.(4.03 x 3.03 cm)
106.667
1024 x 768 (0.8 MP)2.541 x 1.906 in.(6.45 x 4.84 cm)
170.667
1280 x 960 (1 MP)3.176 x 2.382 in.(8.07 x 6.05 cm)
213.333
1600 x 1200 (2MP)3.97 x 2.978 in.
(10.08 x 7.56 cm)266.667
2048 x 1536 (3 MP)5.082 x 3.811 in.
( 12.91 x 9.68 cm)341.333
2272 x 1704 (4MP)5.653 x 4.228
(14.32 x 1074 cm)378.667
2560 x 1920 (5MP)6.352 x 4.764
(16.13 x 12.1 cm)426.667
As can be seen by the PPI numbers shown above, in the case of our test images,only the 5-megapixel image would be able to produce a 6 x 4 inch print without anyresampling. If instead of a dye sublimation printer an inkjet type printer was used, alower resolution would yield a larger image. However, this printer has a fixed DPI,much like commercial digital photo printers.
The images below are scans made of the test prints. The scans, measuring 400 x300 pixels at 300 PPI, where done of the same section of the prints. (It is worthnoting that scanning produces a lower resolution image.) The images were thensaved in JPEG format with the quality set to 60% to avoid creating further artefacts:
To produce a 6 x 4print at 403 PPI, the640 x 480 image needsto be resampledconsiderably. The result
is an image thatexhibits a lot of artefacts, and onlysome of the words inbold are legible.Printing this imagewithout resampling ispossible, but althoughit appears more crispand has fewer colourartefacts, it still lackssufficient detail to be
readable.
1600 x 1200:
Surprisingly, the 2-megapixel image isquite acceptable, evenafter the resamplingnecessary to bring it upto the printer'sresolution. Some colour
moires can be seen,but obviously, a 2-megapixel resolution isalready sufficient toproduce an acceptable6 x 4 image with thistype of printer.
2048 x 1536:
If 2 megapixel isacceptable, then 3should be even better.Indeed, the 3-megapixel image showsthat the minimalresampling requiredpermits a goodsharpness. Finally, wesee even less colourmoires than in the 2megapixel image.
At the 5-megapixelresolution, the imagecontains more datathan is required for a 6
x 4 print at 403 DPI.The text is sharp andvery legible. There areno colour moires. Toproduce this image, theprinter driver actuallydown samples thedata, a process thatcan have the effect of augmenting the
perceived resolution byeliminating any
artefacts that might bepresent in the image.
To the naked eye the text in the printed photos is small (approximately equivalentto a 5 point face), but completely legible in all but the print made from the 640 x480 pixel image.
As explained at the beginning, the point of the exercise is to illustrate the fact thatthe higher the resolution, the better the print quality. The Sony DPP-EX5 has aresolution that is commonly used for commercial digital printers, the machines thatare used to produce prints in stores and at photo kiosks. The results shown hereclearly demonstrate that to obtain a high quality 6 x 4 inch print from such amachine, a minimum resolution of 2 megapixel is required. Moreover, it also shows
quite clearly that an even higher resolution is better.
For anyone purchasing a digital camera with the intention of replacing their filmcamera, and for anyone who intends to either print, or have the photos printed,resolution should be the foremost consideration when making a selection.
Although there has been information concerning digital zooms on megapixel.netfor nearly 4 years, digital zooms have continued to evolve. Moreover, to this day,one of the more frequent questions we get asked remains "what is a digital zoom?".
The concept behind the digital zoom is actually very simple, but often, the way it ispresented as an extension of the optical zoom, creates confusion. Examples of thisare common: a 10X optical zoom suddenly becomes a whopping 60X zoom, as a 6Xdigital zoom is tacked on to the actual focal length of the lens. This puffery maymake the numbers more impressive, but are quite misleading for the uninitiated.
The digital zoom is primarily a cropping system and not as labelled: a "zoom". A realoptical zoom is a lens which can bring the photographer closer to the subject, muchlike a telescope can. A digital zoom on the other hand is merely a system that cutsthe central part of a digital image in such a way that it corresponds approximatelyto the field of view that a longer optical lens would show.
Here is a simpleexample. The photoshown at right iscaptured with anoptical zoom which, inthis case, correspondsto a 42mm focal
length.
The original image iscaptured with a 6megapixel camera,which yields an imagesize of 3024 x 2016pixels, and is reducedto a size of 400 x 267pixels, to show here.
The subject at thecentre of the frame is
some distance away,but if we crop theoriginal image to thesize shown above, a400 x 267 pixel frameallows us to show thesubject in the samephysical space on thispage.
This is exactly what thedigital zoom does. It
simply crops theimage. At this stage,the image qualityremains the same as itwas in the originalimage.
Obviously, a system that allows this type of cropping to be done directly with thecamera can be useful, but most digital zooms take the process one step further.Indeed, what happens afterwards is critical to the image quality, either it remainsthe same as it was in the original full-size frame, or it gets degraded. Currently,there are 2 very different types of digital zooms. One type interpolates the
image back to the size of the image the crop was taken from, the other leaves thecropped image as is. We'll start by looking at the first type of digital zoom.
With most digital zooms, the cropped section from the full size frame is interpolatedto create an image that has a horizontal and vertical size that is same as the imagesize currently selected on the camera. The interpolation process that takes thatsmall cropped section and brings it up to a much larger size involves the use of complex algorithms, and takes place in the camera. The sizing up process is doneby adding pixels in between the existing ones, matching their colour. Since only theoriginal pixels in the cropped section recorded actual information, the interpolationprocess cannot add detail.
Since the pixels thatmade up detail for theimage are multiplied,the inevitable result isthat the imagebecomes coarser. Infact, the larger the
image size increaseachieved byinterpolation, thegreater the degradationof the image.
With our exampleabove, if we were totake the crop we madeand increased its sizeto match the size of theframe it came from, wewould need to blow itup by 756%.
Shown here is a comparably size section (400 x 267 pixel) taken from the crop shownabove which has first been interpolated up to 3024 x 2016 pixels, resulting in a severe
loss of detail.
Although most digital zooms don't go to this extreme — a 756% magnificationwould be equivalent to a 7.56X digital zoom— the principle is nevertheless thesame, and the image quality invariably decreases. Commonly, digital zooms apply alesser interpolation, as otherwise the image quality declines precipitously.
With the majority of cameras, the digital zoom crops from the optically magnifiedimage. In other words, the crop is made from what the camera captures at the fulltelephoto setting of the lens. The examples below show the process:
The entire image shown above and to theleft is what is captured with the camera'soptical zoom. The yellow frame isequivalent to the area a 3X digital zoomwould select from this frame.
In the image above on the right, thedigital zoom "discards" the rest of theimage, shown by a yellow overlay, andselects the centre. The last photo, shown at left, is the
resulting digitally zoomed image. Thecropped section has been interpolated by300%, resulting in an image that has thesame dimensions as the one it camefrom.
The other type of digital zoom has appeared more recently, and is often referred toas the "Smart Zoom". The Smart Zoom is an evolution of the concept, and becamepossible as the image resolution of cameras increased. Nowadays, 4 and 5megapixel image sizes are common, and this allows a smaller, but still useablesection of the full resolution to be used without interpolation. With this systemhowever, for the Smart Zoom to be available, the user must select an image size
inferior to the camera's maximum image size.
With all cameras, when a smaller image size than the full resolution of a camera isselected, the image captured by the sensor, whatever that may be, is "downsized"by the camera to match the selected image size. The downsizing process can bethought of as the opposite of interpolation, the image size is reduced by extractingpixels (data) from the full size image resulting in the desired image size. In this casehowever, although data has been extracted from it, the image generally retains asharp and detailed appearance.
It is this process theSmart Zoom takesadvantage of, by using
the full resolution of the sensor andcropping the centralpart of the image itcaptures.
As long as the imagesize is less than thefull resolution of thesensor, the SmartZoom can work fromthe full image, avoid
any interpolation, andthe attendantdegradation of theimage quality that theinterpolation processentails.
Therefore, starting witha camera that offers a5-megapixel resolution(a full resolution of 2592 x 1944), whenthe the camera is set
to capture a 640 x 480pixel image, the fullsize image is internallydownsized to 640 x480. If the opticalzoom is used, up to itsmaximummagnification, thecaptured photo isdownsized to 640 x480. But, when theSmart Zoom is used,
the difference betweenthe selected image size(640 x 480) and theactual resolution of thesensor (2592 x 1944),
allows capturing animage that seems tohave been taken withan even longer lens,but is simply croppedand at most downsizeda slight bit.
One could assume that this same principle holds true with all digital zooms. As longas the image size is set to a smaller size than the full resolution of which the sensoris capable, the camera will reduce the degree of interpolation, or avoid it altogether.However, such may not be the case. Just as likely, the camera will interpolate thedigitally zoomed image, and then re-size it downwards to save it; in effect "blurring"the digitally zoomed image through interpolation first, and then "re-sharpening" itthrough the downsizing process. Not quite the same as a Smart Zoom.
Conclusions:
Until the arrival of the Smart Zoom, the Digital Zoom was probably the most
misleading, and frankly, useless element found on a digital camera. As it wasoriginally implemented, the digital zoom simply degraded the picture quality.Moreover, it was regularly used to boost the apparent capability of a camera, whichonly served to confuse those unfamiliar with its function. This is changing. Mostnewer cameras — with some exceptions — are not marketed with the digital zoomlumped in with the optical zoom, and others are adopting the Smart Zoom concept.
The ISO setting on a digital camera is a measure of its sensor's sensitivity to light.Low values indicate a lesser sensitivity to light, high ISO values indicate a greatersensitivity to light.
With all cameras however, an increased sensitivity to light comes at a price, and thisis true to a greater or lesser extent with all types of digital cameras, ranging fromthe least to the most expensive models. As the sensitivity of the sensor augments —a process akin to increasing the volume of a radio — electronic noise starts toappear, the visual equivalent of the sound distortion that can become audible whenthe radio's volume is turned way up. Put simply:
LOWER ISO SETTINGS HIGHER ISO SETTINGS
• less noise • more noise
• cleaner image • less detailed image
• requires more light, or • requires less light, or
• larger (bigger) aperture, or/and
• allows a smaller aperture, or/and
• longer shutter speed • allows a faster shutter speed
Usually, selecting an ISO setting is based on two factors, first the ambient light ; andsecond the shutter speed necessary to capture the image. Occasionally a thirdelement, the need for a specific aperture, can come into play, but it is less often adeciding factor for choosing a higher sensitivity.
As a rule, to maintain the highest image quality possible, preference should always
be given to using as low an ISO setting as possible. For instance, catching a sharpimage of a moving subject on a cloudy, or overcast, day will require a shutter speedabove 1/125 sec. But, at a sensitivity setting of 50 or 100 ISO, that may not bepossible even at the widest aperture of the camera. In this case the only choice is toincrease the sensitivity of the camera so it can use a faster shutter speed, andfreeze the movement.
However, any increase made to the ISO setting should be gradual. Immediatelyincreasing it to the maximum when it may not be necessary would be a bit of asledgehammer approach to the problem. What is needed is a step by step approach,first trying at the next setting up from the current one, and seeing if the shutterspeed increases sufficiently to achieve the desired goal, and then increasing thesetting further only if necessary. That way, noise is kept to a minimum, and theimage quality is the highest possible under the circumstances.
Advantages and Disadvantages
Noise and sensitivitywork in lock-step.Noise tends to show upin the shadowed partsof the image first, andalso in areas withnearly uniform colourtones.
In the examples below,we've selected a smallsection of the imageshown at right that is a
shadow area, an areathat is back-lit. Thesection we haveselected here is not inthe deepest shadows of the frame, but it falls inthe zone of focus,which is where noisewill be mostnoticeable.
The examples shown above demonstrate that for a given aperture, in this case f3.2,the increase in the ISO setting of the camera produces an increase in the shutterspeed. In this case, the shutter speed moves from 1/5 all the way up to 1/40 sec.
In practical terms this can be useful. When the camera is set at 50 ISO, a 1/5 secshutter speed requires a tripod, while with the camera set to 400 ISO, it can behandheld quite reliably at a shutter speed of 1/40 sec.
While this example is using low shutter speeds, equivalent gains could be made inshutter speeds but in a higher range if the subject was brighter.
If the gain in noiseshown above is quitenoticeable in theshadowed parts of theframe, they are muchless obvious in thebetter lit areas of theimage, such as in azone that falls
approximately in-between the lightestand darkest points(yellow rectangle in theimage at right).
In other words, thegreater the amount of light, the lesser theimpact of the noise, ascan be seen in thephotos below:
As can be seen in the images above, noise remains visible in this better lit area, butit is less obvious. In fact, in this example, the 200 ISO image is actually quiteuseable, and has the advantage of allowing a shutter speed that is four times as fastas the one available at 50 ISO.
The fact that the impact of noise on the image decreases with the amount of lightbecomes even more obvious with photos taken in really bright light.
In the example shownat right — a generallysimilar subject to theone above — thesubject is in directsunlight. We haveclipped a section of the
image (red rectangle)to illustrate the pointthat with a brightly litsubject, a very fastshutter speed can beachieved with minimalapparent noise gain. Itis worth noting thatwith this example, the400 ISO setting is infact causing someoverexposure as thecamera is at its fastestshutter speed andsmallest aperture forthe focal length.
In summary, while high ISO settings always cause an increase in noise, they alsoallow the user to push the camera to higher shutter speeds, which can be veryuseful.
Remembering that noise is most apparent in the shadows and uniformly colouredareas when the ISO is increased, the subject should be evaluated so that areas thatmight be prone to noise are identified. Then, re-framing so as to eliminate as muchof these areas as possible from the image will greatly lessen the visible noise.
Under low light conditions, noise always becomes much more obvious, and thereforeincreasing the ISO sensitivity is likely to degrade the image quality considerably.Somewhat counter-intuitively, when shooting in low light it is better to stabilize thecamera, and use as low a setting as possible to minimize noise.
Why compress?
Let's first start by pointing out that the subject of image digitisation fills manyvolumes and is quite complex. This article is intended to give a simple overview of what compression is and how it works. For those so inclined, resources are availableon the Internet, and in many books, which can provide an in-depth look at a very
complex subject.
The larger the image, and the more precise the sampling process, the larger thefinal digital file will be. To make the use of digitised photographs more practical —be it for transmission over networks, or for storage on a disk — algorithms havebeen developed to reduce the amount of data that is used to define the image.When the process is reversed, the digital image is restored. Compression algorithmsare particularly useful when storage space is at a premium, or when datatransmission speeds are critical. To achieve real savings in the file size, manycompression systems sacrifice some of the information the file contains. The objectis to make a compressed version of the image, so that once restored it is asindistinguishable as possible from the original image.
Digitisation
When a photo is digitised, itscolours are sampled andconverted to binary format.The smallest image elementsampled is a pixel. A digitisedimage can be better imaginedif it is thought of as a map,where the informationconcerning the colour value of any given pixel is retained as
an X–Y (Cartesian) co-ordinates on the map. Whenthe map is converted back toan image, the pixel regains itsposition and colour inrelationship to the other pixelsmaking up the image.
The different types of image compression
Many different algorithms have been developed to compress file sizes. For our
purpose here, there is little point in considering all of them, but we will define twobroad categories: lossy compression and non-lossy compression.
A non-lossy compression scheme encodes the data so as to express it in a morespace-saving way. An example of such schemes is LZW compression, an acronym
for the names of its inventors: Lempel, Ziv and Welch, owned by Unisys. Generally,LZW can compress a photo down to a ratio of 2:1, and sometimes a bit more. Mostnon-lossy systems can only offer a small savings in the file size; However, when afile is saved in this way, it can be restored accurately, without loss in either imagedetail or quality. Lossy compression, on the other hand, discards part of the dataentirely. When the image is restored, it has lost some of the information containedin the original file. As we will see later, in most cases this is is not as critical as it
sounds.
Lossy compression, depending on the level of compression used, can reduce the sizeof the image file to a ratio of 10:1 and even sometimes 20:1. In digitalphotography, one lossy compression method has gained favour over all others, amethod devised by the Joint Photographic Experts Group, and whose initialshave become the name of the compression: JPEG.
How it works:
JPEG was created specifically for the transmission and storage of photographicimages. As a lossy compression algorithm it is made to remove a varying amount of the data that originally made up the image. JPEG compression is designed to takeadvantage of a particular aspect of human visual perception: the fact that weperceive small colour changes less accurately than we perceive small changes inbrightness.
JPEG compression works in three main stages:
1. transformation
2. quantization (the lossy stage)
3. encoding
The first step — transformation — changes the data so it expresses the image in
terms of chrominance (colour values) and luminance (brightness). This step iscritical for the next one: quantization.
Quantization is the step thatactually discards some of data,so that the data set needed todefine the photo is smaller.
The entire image is analysedby areas of 8 x 8 pixels, whichmake up blocks of 64 pixelseach. Through a complexmathematical process the
chrominance found in theseblocks is "averaged" so that itrequires less data to expressthe values in the block.Expressed simply, this meansthat the colour variations thatexisted in the original imageare lessened.
Finally, an encoding step, which uses a process similar to non-lossy compression, isapplied to the data resulting from the quantization so as to use even less space.
When the file is read back, the process is reversed, re-creating an image that issimilar to the original when seen through human eyes.
JPEG compression can achieve very high compression factors. Some of the imagesbelow will demonstrate this. But, as a lossy compression system, it means that
when examined closely, the differences between the original and the JPEG versioncan be observed. The images below are magnifications and are used to demonstratethe effect of compression. At their normal scale the changes made by the JPEGcompression would be much less noticeable.
Some examples:
The image to the right exemplifies whatthe quantizing step of the JPEG processdoes. Faint square blocks are visiblethroughout. They show the areas in whichthe quantization process took place. Theprocess reduced the chrominance in theseareas, or in other words, the variations insmall colour changes throughout theimage.
The result is a loss of image detail orsharpness. This can be hard to detectwhen the image is seen at the scale at
which it was intended to be seen.However, a slight change of texture andcolour detail is usually visible.
NOTE: the images both above and below are in JPEG format, but at a low compression.However, because they are taken from magnified views, they still clearly exhibit whatthe effect compression.
The uncompressed image:
Here, the image to the right is a 200%magnification of the same wooden objectas shown above. A much greater amount
of subtle colours is immediately visible.Also noticeable is a smoother edge nextto the black portion. With digital cameras,the option to save uncompressed photoscan be valuable when these contain a lotdetail. More and more, new digitalcameras offer the possibility of storingphotos in an uncompressed format. Withthe cost of memory cards dropping, andmemory capacity increasing, it isbecoming advantageous to have thiscapability.
The effect on colour
Below, the same photo below has been saved in two formats. One is verycompressed using JPEG compression, the other uses no compression. To show thedifferences both images were opened in a photo editing program and magnified300%. A screen capture utility was then used to create these images. To avoidadding further artefacts, they were then saved in GIF format. The GIF compressionreduces the colour palette of the image to reduce its file size. Since this imageactually contains few colours (blues, greens and black), the GIF format has aminimal impact on its appearance.
Readers of megapixel.net may have noticed that we tend to stress the availabilityof a "no compression" option when we review cameras that offer it. The reason issimple: we have found that looking at images as they are, generated by the lensand sensor of a camera, and without any compression, tells us much more about
the camera's quality than looking at compressed images.
While compression offers many advantages in digital photography, the option to get precisely what was captured by the lens and sensor of a camera should not beignored. Not only does it help in deciding whether the camera is able to generatethe image quality a potential purchaser requires, but some images, depending onthe subject, can show a great deal more detail when left uncompressed. If only forthese two things, a "no compression" option should not be discounted.
Metering and metering patterns
All cameras use an exposure meter. The exposure meter evaluates the brightness of the subject at which the lens is pointed, and sets the aperture and shutter speed of the camera so as to capture the image correctly. In other words, the basic ideabehind metering is to ensure that the image is as close to reality as possible andthat it accurately reflects the way we see the subject.
The metering systems that have been developed over the years to evaluateexposures have gotten progressively more and more complex, and nowadays, mostcameras rely on one of two basic systems: averaging or centre-weighted. Of these,centre-weighted was the first to be developed, so we'll look at how it works first.
Centre-weighted metering
As the name implies, a centre-weighted metering pattern yields exposureparameters that are "biased" towards what is at the centre of the frame, usually
where the focus point or primary subject of the image is located. Few centre-weighted systems are precisely identical, as every manufacturer develops their ownway of implementing centre-weighted metering, varying the diameter of the centralzone, and the importance it is given in relationship to the rest of the frame.
This aside however, by the way it works, the exposure all centre-weighted systemsyield are influenced by the brightness of the subject that falls into the central area.An example of this is shown below, which clearly shows that a slight variation inwhat is at the centre of the frame can have a noticeable effect on the overallbrightness of the exposure:
In the image above, note the balancebetween the darker hull and the whitesuperstructure. To the centre-weighted meter,the darker hull is a little bit less dominantthan the white superstructure. It adjusts forthis and the result is a slightly dark image.
This time, the image above contains a bitmore of the darker hull. The result is anadjustment in the settings of the camera toincrease the brightness of the exposure so asto capture more detail in the darker area, andthe image is therefore a bit brighter.
With the examples above, it is also worth noting that both images are generally verywell exposed, albeit with minute variations. These variations are one of thestrengths of centre-weighted metering. With a bit of practice with a camera'scentre-weighted meter, it is possible to exercise a lot of control over the imagesimply by being aware that slight variations in the overall brightness result fromwhat is placed at the centre of the frame.
Uses: Centre-weighted metering is fine for most subjects since it meters the entireframe. With side or back lit subjects, the centre-weighted metering can yieldexposures that take into account the variations in lighting.
Spot metering
Spot metering concentrates the metering of the subject on a small area. Originally,spot meters simply metered a small circular area at the precise centre of the frame.The idea being that spot metering was to measure the precise point of focus, whichin many cameras, used to always be dead centre of the frame. Over the years,focusing systems became more complex, and many are now able to find one ormore focus points in a frame. In turn, spot metering is often tied to a single focuspoint, even if the focus point can be displaced to a part of the frame other than the
centre.
Spot metering has a profound effect on the exposure. This is exemplified by theimages below:
Spot metering, Program modeAperture: f5.6, shutter speed: 1/340 sec.
Spot metering, Program modeAperture: f3.0, shutter speed: 1/160 sec.
With the image above, the spot meter isplaced on a point that is highly reflective, andwhich is directly lit by the sun. The result isan exposure that has captured thesuperstructure clearly, but not the hull. Toachieve this, the camera closes down the
diaphragm, and uses an aperture of f5.6 incombination with a high shutter speed tolimit the light striking the sensor.
While in the image above, the spot meter isaimed at the darker part of the hull.(Compare the area within the yellow circlewith the image at left.) The resultingexposure burns out the bright whitesuperstructure of the ship, but clearly images
the hull which is in deep shadow in the imageat left. Note that the shutter speed hasdropped, and the aperture is wider (f3),allowing more light to strike the sensor.
The value of the Spot meter is its precision. Once a specific point is metered, it willbe exposed properly, regardless of the surroundings. The precision of the spotmeter is precisely what the most advanced metering systems take advantage of, bycombining multiple spot metered areas to calculate the best exposure for the entireframe.
Uses: the spot meter is useful with macro photography, or when the point of the
photo is to capture the subject, even at the expense of anything else in the frame.
Averaging systems, also called pattern metering, matrix, and segmented metering
An averaging system takes dozens, and sometimes hundreds, of simultaneousreadings throughout the frame, averaging the results to obtain the best combinationof aperture and shutter speed.
The segmented metering system analyses thebrightness of the frame in numerous areas. Inthis example 64 discrete areas of the frameare measured.
64 Segment metering, Program modeAperture: f4.0, shutter speed: 1/170 sec.
In its simplest form, the segmented systemcalculates the brightness for the subject froman average of all the readings that weretaken, and thus determines the aperture andshutter speed to use.
In principle, exposures determined byaveraging generally produce images that areless prone to having parts of the image overor underexposed. However the way theexposure is calculated can, with somesystems, be much more complex than thisexample shows.
As mentioned above, a few of the segmented metering systems combine theinformation they gather from the frame with a database of typical compositions.This process is sometimes referred to as scene recognition.
With these systems, the data gathered at each segment is compared to an internaldatabase of readings gathered for many different scenes and the system tries tocorrelate the results of the metering to a scene in the database. If the metered datais "matched" to a specific type of scene, then the system is able to tweak theexposure further. For instance, should the system recognize that the frame about tobe photographed most likely involves snow, it may adjust the exposure to take intoaccount the fact that snow is highly reflective, and purposefully force a slight
overexposure to ensure that the snow in the image comes out white instead of lightgrey.
Uses: an averaging or segmented metering system is intended to be used as theprimary metering system. It is ideal for subjects and scenes that are well andevenly lit.
Colours vary with the source of light
The white balance setting of a camera relates exclusively to the colours that arecaptured. White balance is critical to a digital camera as it establishes the startingpoint for all the colours it will record. The purpose of white balance is to ensure thatwhatever the source of light, the colours of a subject photographed under that lightwill be reproduced as faithfully as possible.
By default, most digital camera's white balance systems rely on an Auto whitebalance, a series of algorithms that divide the image into many segments toevaluate the light source, and then compensate for it by shifting the colourspectrum of the image according to that light source. The light colour evaluationsystem works by comparing the data gathered against the known colour shiftsimparted by different sources of light, such as sunlight, or fluorescent. Each cameramanufacturer uses their own sets of algorithms to calculate the proper whitebalance for an image; and in most cases, the resulting colours are quite accurate.On occasion though, for a variety of reasons, the auto white balance can be in error,resulting in an image whose colours are not as accurate as they should be.
An incorrect white balance setting results in colour shifts, and these can be quitedifficult to correct once the image has been finalized by the camera and saved in aformat such as JPEG, or TIFF. This difficulty in correction, should the auto whitebalance be in error, is one of the advantages of the RAW image formats that areoffered on some cameras. A RAW format allows correction to an improperly setwhite balance relatively easily, usually with the same software that is provided to
convert the image into a more common format. On the other hand, correcting thewhite balance of images saved in other less flexible formats — JPEG for instance,which has deleted unnecessary colour information to reduce file size — is sometimeshopeless.
White light is composed of all colours, but white light, be it natural or artificial, canvary in its purity. We all know for instance that the light of the setting sun is much
more red than the light of the sun at noon, and this is reflected in the way we seecolours. Similarly, we can also tell that the light produced by a cool whitefluorescent tube is a bit more green than the light produced by a standardincandescent bulb.
To demonstrate this, aseries of images weretaken using the Autowhite balance of acamera, and switchingthe light sources foreach shot.
In the still life shownhere, the background iswhite, and can be usedto see the impact of the light source. Thefirst photo (at left)shows a correct whitebalance, and can serveto evaluate the others:
Auto white balance:
with incandescent light(100W bulb)
Auto white balance:
with a Fluorescent Desk Lamp(13 Watts U-shaped bulb)
As can be seen in the images above, the auto white is not always precise.Interestingly, with these examples, the auto white balance fares much better whenthe light source is the cool white fluorescent as that source of the light is moreeasily identifiable, something that appears to be the case with a number of cameraswe tried.
Compensating for the light source
This next series of photos show the result of manually adjusting the white balanceto the ambient light. Most cameras offer presets for common sources of light, andthese generally provide good results. The same subject re-photographed, using thesame light sources as were used above, but adjusting the camera's white balance tothe preset offered for that specific light source :
Light source: 100 Watt bulb
White balance Preset:Incandescent or Tungsten
As can be seen, using the Incandescent or Tungsten — same concept — setting forthe white balance results in a correct image.
With the Desk Lamp Fluorescent light however, rectifying the colours proved to bemuch more complicated. Many cameras offer a variety of settings for differentfluorescent lights, usually 3 settings, daylight fluorescent, warm white fluorescent,and cool white fluorescent. Each is tried in turn below:
As can be see above, none of these yield natural colours. In fact, the FluorescentDesk Lamp tube seems to have a colour temperature that is closer to anincandescent bulb (below). Although even then the result was not as good as itshould be and the image retains a very noticeable yellow cast:
Light source: Fluorescent Desk Lamp
White balance preset:Incandescent/Tungsten
When none of the preset white balance settings are perfect, the only option, if thecamera is so equipped, is to set the white balance for that specific light source.
That setting, referred to as a "user-set", or "spot white balance", is explainedfurther on.
In this test, the problem experienced with the Fluorescent Desk Lamp does notoccur with the standard cool white fluorescent. Setting the white balance to thatspecific preset yields an accurate image:
During the day, the sun is our primary source of light, but the colour of its lightchanges according to the time of day, and the weather. Generally, most Auto white
balance systems are quite reliable under sunlight, be is direct or indirect, andsetting the white balance to Sun or Daylight has very little impact. This can be seenin the examples below, both captured in direct sunlight:
Light source: direct sunlight
White balance set to Auto
Light source: direct sunlight
White balance set to Daylight/Sun
As these two photos show, whether the Auto white balance is used, or the Sunwhite balance preset, the results are similar. In fact, the Sun or Daylight presetsoffered on many cameras are best used only when the sun is directly overhead.
The Shade preset in most cameras is also designed for fine sunny weather, but isspecifically intended for subjects that are not directly lit by the sun, a situation thatmakes colours bluer, and which this preset rectifies.
When the weather is cloudy, the Auto white balance may yield photos that have apronounced bluish tint, particularly in shadow areas, and if a setting is available forCloudy the image will generally show more faithful colours.
Focal length is defined as "thedistance between the rearnodal point of the lens and thefocal plane, when the focus isset at infinity"¹. The rear nodalpoint of the lens "is where therays of light appear to have
come from, after passingthrough the lens"².
The focal length of a lens isexpressed in millimetres. Thefocal length of the lens alsodetermines the field of view:how wide, or narrow theview through the lens is.
The photo at left shows a 35mmcamera fitted with a 50mm lens. Inthe 35mm film format, lenses withfocal lengths in or around the50mm mark, are generally referredto as normal lenses. This isbecause these lenses show objectsand scenery, at approximately thesame scale as is perceived by theunaided human eye.
The photo on the right showsthe field of view of a 50mmlens. The image shown here istaken through the viewfinderof the camera.
The scale is very similar towhat an observer would seewith the naked eye while
Now, we see thesame camera, butthis time, fitted witha 200mm lens. Thelens magnifies theimage reaching the
focal plane and inturn, narrows thefield of view, justlike binocularswould.
Again, the photo on the leftshows what can be seenthrought the viewfinder of the
camera.
Since the image isconsiderably magnified, weonly see a small portion of what was visible in the 50mmimage, but that portion nowshows much more detail thanwas visible previously.
The Optical Zoom
A zoom lens is a lensdesigned so that its focallength can be varied overa predetermined range.The image is opticallymagnified and, depending onthe zoom's setting, will showa larger, or smaller field of view. The variability of thezoom lens allows it to
With a digital camera, theoptical zoom does not changethe image size, or theresolution. The number of pixels used to describe theimage remains constant.
Therein lies the difference withthe digital zoom.
In this example, the photo istaken with the camera's zoomset to the 38mm position,offering a broad field of view.The image's resolution is 1600x 1200 pixels.
Now, we zoom in to 115mm,maintaining the camera in thesame exact position. Theimage still has the sameresolution as the previous one:1600 x 1200 pixels. Similarly,the number of kilobytes usedto store the image file withoutcompression, will be the sameas the photo above.
The optical zoom has broughtthe subject "closer", showingmore detail than was visiblepreviously, as if the camera
had been moved physicallycloser to the subject.
The Digital Zoom
Digital cameras equiped with asingle focal length lens,usually offer the possibility of mimicking a zoomed image.The system is called a digitalzoom.
With single focal lengthcameras, the process works bycapturing only the centralportion of the entire imagereceived by the sensor.
In a real sense, the digital zoom is really only a cropping tool, since it cuts off theparts of the image that would be out of the field of view if a longer focal length lenshad been used.
The image above shows the "zoom" possibilities. For example, if the real image has
a size of 1600 x 1200 pixels, a 2.5X digital zoom would capture a smaller imagemeasuring only 640 x 480 pixels, taken from the centre of the frame. In theillustration above, the white rectangles indicate other possible digital zoom options.
Inherent in a digital zoom, is the fact that the numbers of pixels used to capture the
image is the same, as the number of pixels representing the same area on theoriginal, non-zoomed image. Therefore, the digital zoom image is either smallerthan the image it was cropped from but has the same definition (the same thenumber of pixels are used to represent the same area), or, if the image is re-sizedby interpolation to be of the same size as the non-zoomed image, it exhibits a muchlower definition.
Since the physical number of pixels that captured the image is always constant,resizing the image by interpolation has a serious impact on the quality of the imageitself. The best way to demonstrate this is to look at the examples below:
The image shown on the rightis an un-cropped reduction of an original photo whichmeasured 1600 x 1200 pixels.The image is captured with asingle focal length lens of 40mm.
To show the effect of a digital
zoom, we selected a 640 x 480pixel portion from the centralpart of the original frame(white rectangle). Then, were-sized this new smallerimage to the samemeasurements as those of theoriginal photo: 1600 x 1200pixel.
Resizing the smaller cropped image has the effect of lowering the definition.
We cropped each photo (represented by the smaller yellow rectangle in theillustration above), so they could be placed side by side below.
This is a 280 x 210 pixel portion of the original image,showing the definition of the non-zoomed image. Thedefinition exhibited here is normal for a distant object,
imaged by relatively few of the sensor's pixels.
This is a 280 x 210 pixel portion of the zoomed image.The interpolation of the original image data has causedthe photo to become quite blurred, as the interpolationalgorithm "invented" pixels to increase the image size.
What is important to remember, is that the very same thing can be accomplishedwith just about any photo editing program. A photo can be cropped to what ever isrequired, with greater flexibility than the digital zoom provides, since any given areacan be selected.The digital zoom on the other hand, can only use the centre portion
of the entire frame.
If there is an advantage to the digital zoom, it is that since the camera meters onlythe "zoomed" section of the image (a smaller section at the centre of the frame), itcan quite often generate a better exposed image for that particular area, than if the
exposure had been determined based on the larger image.
Pixels Count
The normal expectation is that the sharpness of an image depends on the precisionof the optics and of its focusing system. However, this is only part of the story. In adigital camera, there is another critical component: the sensor, usually a ChargeCoupled Device (CCD).
The CCD is made up of
groupings of red, greenand blue pixels. Theseare the elements thatregister the image. Theimage is focused ontothe CCD and recorded.
If we assume that wehave a perfect lens,and that whatever thislens sees is tack sharp,then the image pickedup by the CCD shouldalso be tack sharp. Yet,this is not the case.The sharpness of theimage is directlyproportional to thenumber of elementsused to record it.
Common types of resolutions
To understand how a CCD's resolution impacts the overall image sharpness, let's, asan example, compare three CCD resolutions:
640 (H) x 480 (V) pixels,
1280 (H) x 960 (V) pixels,
1600 (H) x 1200 (V) pixels, or 2 megapixels.
What these numbers represent are the numbers of pixels, horizontal and vertical,that represent the imaging area of the CCD. These numbers are used to define theresolution of the sensor.
For each of the camera reviews in megapixel.net, we use one specific building toshow the possibilities offered by the lenses of different cameras. We have takensome of these images to demonstrate the relationship of resolution, to imagesharpness, by sectioning off a 300 x 300 pixel part of the images.
Keep in mind, here we are only comparing what is imaged by a group of 300 x 300pixels at different resolutions. We are not concerned with the quality of the image.The focal lengths used for each photo are generally similar, all falling around the
In this example, the entire building isimaged in a frame measuring 640 x480 pixels. This represents a total of 307,200 pixels.
Cutting a 300 x 300 pixel section of the frame, we can see a good portionof the building. What is importanthere is that this large section of thebuilding, was imaged by a mere90,000 pixels (300 x 300).
1280 x 960 pixels.
Using a camera equipped with a CCDoffering a resolution of 1280 x 960,we can see that our 300 x 300 pixelsection contains a lot less of thebuilding. Another way of looking at it,is to say that since there were manymore pixels available, our 300 x 300section only had to capture a small
corner of the building. This alsomeans that the overall imagecaptured more detail.
A frame measuring 1280 x 960 pixelsyields a total of 1,228,800 pixels.This number is commonly rounded upand called a 1.3 megapixel
resolution (1.3 million pixels), or 4times as many pixels as the 640 x480 photo above.
Now we see what a resolution of 1600 x 1200 can capture. Again, thesection shown on the left is 300 x 300pixels. Notice there is less differencein terms of the size of the area
captured, than there was between thefirst two examples. A CCD of this typeprovides 1,920,000 pixels, 6.25times as many pixels as the firstexample, but only 1.5 times thenumber of pixels as the the CCD witha resolution of 1280 x 960.
Still, even with this relatively modestincrease in the total number of pixelsover the previous image, we can seethat the definition of the trees in the
background is improving.
To make the point that the number of pixels is critical to the amount of detail contained in an image, and inturn to the level of "sharpness" weperceive, we have scaled the samearea of the photo taken at 640 x 480,as was imaged by the 2.1 megapixelsensor in the example immediatelyabove.
In other words, the section of thephoto on the right is a blow-up of thesmall section on the left. The photo onthe right shows the level of detailcaptured for that specific portion of the building at the 640 x 480resolution.
The CCD—lens relationship.
As we can see, the number of pixels used to make up the image is directly
proportional to the human eye's perception of image's sharpness. The more pixelsthere are, the greater the level of detail captured, and vice-versa, fewer pixels =fewer details.
This does not mean that the lens creating the image on the CCD's surface isunimportant, in fact, quite the contrary.
In general, lenses are difficult to build because, as they bend the light to refocus iton the image plane, different colours bend at slightly different angles. For example,blue light bends more than red light, resulting in something called a chromatic
aberration. To some extent, all lenses suffer from aberrations, making it virtuallyimpossible to create a perfect lens. Most lenses nowadays use corrective elements
whose functions are to limit, or correct, aberrations as much as possible.
Another difficulty in lens design and construction is that as a lens focuses light onthe image plane, it does so unevenly, creating what are referred to as circles of
"incorrectly focused light from points on a photographic subject that blur intooverlapping circles on the image plane"¹.
In a digital camera, to create a sharp image on the image plane—the surface of the
CCD itself—the circles of confusion created by the lens must have a diametersmaller than the diagonal size of a single pixel. This means that in general,digital camera lenses must be constructed extremely precisely.
Most compact digital cameras use CCDs that are quite small, even though they maypack over 2 million pixels. Diagonal measurements of 1/4", or 1/2" are common.This small image plane area means that the optics needed to resolve an image hasto have a very short focal length. This small size adds to the complexity of thelens design, and generally calls for high quality optics.
What subjects work best at varying resolutions:
What follows here should be considered as general guidelines and not as rules. Butcertain types of subjects are more suited to digital photography with consumer-levelcameras than some others. Generally, most digital cameras have little problemsyielding a sharp and well-defined image with subjects 1 to 5 meters away (3 to 15feet). And, if the optics permit it, they can truly excel at macrophotographywithout the addition of complex accessories such as those required with 35mm filmcameras.
• Distant landscapes, which by their very nature, tend to contain a large
amount of small detail, will generally turn out "sharper" and clearer withcameras that use 2 megapixel, and above, CCDs.
• 1.3 and 1.5 million pixel cameras, will produce acceptably detailed images
with subjects 30 metres (90 to 100 feet) away. After that, their photos tendto become a bit more impressionistic.
Obviously, the greater the amount of detail in the scene to be photographed—foliage for example—the greater the impression will be of a soft focus photo, sincethe smaller image elements will be imaged by only a few pixels.
Selecting the right resolution for one's needs.
Many readers write in to ask which camera to buy. Our reply is generally to suggestthat the writer analyse the kind of subjects they tend to photograph, and decideaccordingly the resolution they need. The rule of thumb is simple: the greater thecomplexity of the subject, the greater the number of pixels required.
Every time information is provided about the lens of a digital camera, the focallength mentioned is also given in 35mm equivalent. While the reason for this maybe obvious to some, it also occasions questions for quite a number of people.
35mm film frames measure 36 x 24mm, and arethe film frame size of the majority of film camerasin use today.
This widespread use of 35mm film has caused manypeople to develop a general idea of the field of viewthat will be visible with various 35mm focal lengths.
In other words, because of its popularity, the 35mmfilm format has become a reference point, muchlike a unit of measure such as the foot or themetre.
For example, a 28mm focal length will capture afairly wide angle; a 35mm focal length noticeablyless; a 50mm lens will provide a near normal fieldof view, a 380mm lens will fill the field of view (seecomparison lower down) with a distant subject.
Put another way, the smaller the number of thefocal length the wider the field of view and,conversely, the larger the focal length number thenarrower the field of view will be; and this holdstrue whether for film or digital.
All focal lengths are measuredin millimetres, whatever theformat of the camera: 35mm,APS, or digital.
The focal length numberindicates the distance betweenthe lens and the focal plane—
the position of the film orsensor. The precise definitionof focal length is "the distancebetween the focus (where theimage is sharp on the focalplane) and the optical centreof the lens". (See graphic at right).
50mm focal length (normal view)
380mm focal length (telephoto view)
With digital cameras, these focal length numbersare usually very small because the image sensorsmost commonly used today are quite small—under
an inch when measured diagonally. To form animage on such a small target, the lens needs to bequite close to the focal plane, hence the short focallength numbers common to many digital cameras.
However, the real reason a 35mm equivalent isgiven, is not because people can't relate to theshort focal lengths of digital cameras, but becausethe "real" focal length on a digital camera—forexample a 6 to 18mm zoom—will not alwayscorresponds to the same field of view on differentdigital cameras.
At the root of this difference is the fact thatdifferent electronic image sensors—the digitalequivalent of a film size—come in a variety of different sizes.
a 2.1 megapixel CCD measuring 0.5 inch diagonally = (1/2")
a 3.3 megapixel CCD measuring 0.55 inch diagonally = (1/1.8")
a 4 megapixel CCD measuring 0.66 inch diagonally = (2/3")
As can be seen, each CCD has not only a different diagonal measurement, but adifferent resolution, which is to say the number of pixels that will form the image.
It is important to note that thenumber of pixels used to formthe image is not related to thethe focal length. In fact, anumber of digital camerashave been produced which,while having different sensorresolutions, are in every otherrespect, similar: same lens,same body, etc. And, if the
sensors used are the samephysical size, the 35mmequivalent of the lens will beexactly the same.
On the other hand, if the lensemployed for each CCD isexactly the same focal length,i.e. 8mm, but the CCDs havedifferent sizes, then their35mm equivalent focal lengthwill be different, as each will
show a greater or lesser fieldof view. (See graphic at left)
So , using a "standardized" way to describe the field of view of digital cameras helpsto simplify everything, irrespective of the size of the CCD in use. And that need for acommon way of expressing the field of view, is why the "35mm equivalent" isgenerally mentioned when a digital camera lens is described.
Burst or Continuous Shooting Modes
An advantage of digital photography is that functions which were reserved to someextent to the professional photographers are now available to anyone. A burst modeis one of these. With film cameras, the burst mode can devour a roll of film inseconds, and to most amateur photographers, the expense and trouble involved indeveloping numerous rolls of film to obtain a few good photos was a bit daunting.
With digital cameras, the burst mode has become part of the list of features that are
considered prior to any purchase. Camera specifications mention rates as "fps",which stands for frames per second, and many buyers weigh this informationcarefully when making their selection.
The speed and number of sequential shots a digital camera captures in a burst is
quite dependent on the amount of internal memory the camera possesses, theimage size selected; and the compression applied to the photos. Cameras with fastburst rates generally have a large amount of "buffer memory", which is used totemporarily store the images prior to processing and storing in the memory card.Professional cameras, depending on the model, offer generous amounts of buffermemory and can hold a number of high resolution—and in some casesuncompressed—images; consumer cameras offer lesser amount of buffer memory
and are generally limited in the number, and the quality (compression) of the photoscaptured. Yet, the principles are similar, and so is the use of the mode.
Memory card requirements
One factor that must be considered when using the burst or continuous shootingmode is that any repeated use of the mode can rapidly fill up a memory card.Unless photo selection is done based on what can be seen on the LCD screen aftereach burst—a slow and inaccurate process—, the purchase of a large capacitymemory card becomes a necessity.
Continuous mode can often be difficult to use effectively
A look at some of the publications renowned for their photos often reflects the useof the burst mode. The photographers of National Geographic, for example, arefamous for expanding great amounts of film so they can pick out the very best shotsof a series. With some subjects, the use of the mode can capture that one perfectimage that stands out from all others.
The same holds true for most non-professional users, the burst mode is used tocapture a quick series of images in the hope that one will be the best of the series,or depict a precise—and desired—moment in time which would have beenimpossible to capture otherwise.
The fact is, many digital cameras have an appreciable shutter lag—the delaybetween the shutter release being pressed and the moment at which the cameracaptures the image. This lag is caused by the time it takes the camera to decidewhite balance, focus on the subject and calculate the exposure settings. This timelag has often been the cause missed photos opportunities, and in some cases theburst mode can be a valuable tool to overcome it.
Using the mode to select the best shot:
Taking photos of fast action requires a bit of planning, not the least of which ishaving the camera properly set, correctly held and focused to obtain the best
possible photos.
Continuous mode is particularly effective when the subject involves action, such asthe playing dogs below. With it, it becomes possible to select the best image of agroup.
Panning describes the movement made when the camera tracks a moving subject.The idea is to move the camera smoothly and keep the lens pointed at the subject.It can be used to photograph the action during a game, or a race. Used inconjunction with the burst, or continuous mode, panning can help to keep thesubject in focus, and can result in a successful photo.
Keeping the subject in a focus area
Probably the most common problemwith using the burst Mode of someconsumer digital cameras is that thecamera autofocuses for the first photo,but not the following ones. This meansthat while the first frame may be in
focus, the subsequent frames may verywell be out of focus, particularly if themotion of the action takes placetowards, away, or even at a diagonalfrom the photographer's vantage point.
If the subject is distant enough, thecamera's "Landscape" mode may beused since it locks the focus to infinity.If the action is nearby it becomes morecomplicated. Unless the camera isequipped with a fast, continuous
autofocus—rare on consumer models—anumber of photos will likely beunusable. A possible remedy is to setthe camera to a small aperture if it isavailable, thereby increasing the depthof field and focus, and pan with theaction.
However, if at all possible, the best option is still to position the camera so as toavoid "out of focus" areas as much as possible. This means positioning the cameraas close as possible to be parallel to the action.
As with everything else, practice and developing techniques for the particular
camera used, will go a long way to improving the results obtained with either burst,or continuous shooting modes.
A viewfinder, separate from the ubiquitous LCD screen, offers a number of advantages. First and foremost, it relieves the batteries of the camera from theheavy drain of the LCD screen; and can considerably increase their life.
Second, it provides a useful alternative to the LCD screen outdoors. LCD screensgenerally require a backlight to be visible and this light can easily be overwhelmedby sunlight, making it impossible to see the image. Alternatives exist, such as
screens that let the sunlight go through and to reflect it back through the image(Sony's DSC-F505 for example); or others that funnel the light through a topopening, and channel it back through the screen. These alternatives help, but boththe colour rendition and contrast of image, is lower than it would be when thebacklight is used.
Separate viewfinders do not have these problems, but they do have others whichwill be detailed later. Viewfinders come in 3 basic types:
Separate from the lens, commonly referred to as an optical viewfinder
Through The Lens, commonly referred to as a TTL viewfinder,
Electronic Viewfinder, also called an Eye-level LCD viewfinder.
The Optical Viewfinder:
Whether the camera's lens is a zoom ornot, the viewfinder is separate from thelens and mimics the view and focal lengthof the lens.
The size of the exit pupil of a viewfinderdictates how clear the image will be, and arelatively large exit pupil is important if eyeglasses are worn since they move the
eye back from the viewfinder. A viewfinderequipped with a diopter correction cancorrect vision and avoid the use of glasseswhen taking photos. But, the correctiononly works with near, or far-sightedness.
All optical viewfinder are placed as closeto the lens as possible to lessen theParallax error.
Parallax error occurs because the lens of the camera and the viewfinder see thesubject from a slightly different position.Over a long distance the error is hard todetect; but over short distances, such aswhen photographing portraits or macros,the difference in the perspectivebecomes magnified and can enormouslyaffect the composition of a photo. This iswhy many digital cameras compensateby turning on the LCD screen when themacro function is used. The screenshows the image the sensor is receivingand avoids the parallax error.
Parallax has always been a problem with separate optical viewfinders, and manyideas have been used to minimize it. Optical viewfinders will generally have "crop"or "parallax" markings, indicating the actual image-taking area, but none of thesesimpler methods are 100% effective. Additionally, optical viewfinders cannot display100% of the image that will be captured and are commonly limited to around 85%
or less of the actual frame. This is why TTL (through the lens) viewfinders weredeveloped.
The Optical TTL Viewfinder:
These are generally used with more expensivecameras, and present the image the lensreceives.
The different systems used to show the imageoriginating from the lens vary in their details, butthey generally reflect, or split, the light comingthrough the lens and direct some, or all of it tothe viewfinder while the image is beingcomposed.
The disadvantage of TTL viewfinders is that they are more expensive to buildbecause of their complexity. This is why they are more commonly found on high-endand professional digital cameras. Furthermore, they commonly require a small LCDdisplay to indicate focusing and exposure, adding to their cost. However, this may
be changing. Hewlett-Packard, working in cooperation with Pentax, recently releaseda camera (HP 912/Pentax EI2000) which uses a TTL viewfinder, but is quitereasonably priced.
Recently, some digital cameras have arrived on the market which use a variation of the TTL viewfinder, but forgo the complex optical systems. Instead, they useviewfinders similar to those found on video cameras: tiny LCD screens which showthe same view and information as the main LCD screen, but in the shelter of aviewfinder.
The Electronic Viewfinder:
The advantages of this type of viewfinder are the
same as for the optical version: they showexactly what will be captured, they are visible indaylight (by virtue of being recessed behind aneyecup), indicate aperture, shutter speed, etc.but, in addition, they can also display the cameramenus—something optical viewfinders cannot do.
The disadvantages of electronic viewfinders can be summarized in 3 points: theyrequire power (unlike the optical, or optical TTL viewfinders); show an overly brightview, just like LCD screens; and are quite coarse in comparison to optical systems.The latter point may be of greater importance since these systems—to date—areunable to show the smaller details in the frame: for instance whether someone'seyes are open, or partially closed.
While no system is "perfect", the general consensus is that of all three the opticalTTL viewfinder is still the best.
How image interpolation works:
For the purpose of this explanation, we will concentrate exclusively on the functionof interpolation in the context of image size. In fact, the process of interpolation is
used in a number of ways, ranging from interpreting the colour information capturedby a sensor to printing an image. Because of this, we will concentrate here on howthe process is applied to cameras and their images.
Interpolation is the process of creating estimated values between existing values.It is used in scanners, digital cameras and image editing software to produce animage with a resolution (the pixel count both horizontally and vertically) greater
than what was captured by the sensor.To put it simply, interpolation"invents" pixels were noneexisted and inserts them inbetween the originals toincrease the image's size.While this may sound likecheating—and clearly incertain cases it is—it can alsobe a critical process. Thegraphic on the right shows asimple interpolation process.
Note that in an actualinterpolation the colours wouldmatch those of the the originalpixels nearby.
There are various methods employed to interpolate an image: Linear, Bilinear,Bicubic and Fractal. All strive to increase the size of an image, while lessening theside effects caused by the process.
Generally, the major problem with interpolation is that it can easily increase thevisibility of any flaws in an image by magnifying them. Bicubic interpolation is
generally considered as the best method, but yet, it may not produce ideal results.The process can induce ghosting—the appearance of white outlines—and jaggiesalong diagonals. In addition, the noise* that is, to some extent, always present in adigitally captured image can become emphasized.
Interpolation works best when it is used in small doses, and applied to a large dataset. An image containing too few pixels is likely to show more degradation than amore detailed one. In either case though, interpolation will not reveal detailsthat were not already present in the original image.
Numerous digital camerashave interpolated resolutionsas part of their possible image
formats. If the camera's nativeresolution is high enough, theinterpolation can be quitesuccessful, and have a minimalimpact on the quality of theimage.
However, if the camera'sresolution is low, or theinterpolation too strong, theresults can be disastrous tothe image quality. These two
images exemplify the problem.The first photo on the left iscaptured using the "real"resolution of the camera.
While the second is capturedusing the interpolatedresolution of the camera.
From an image measuring 640x 480 pixels, the interpolationincreases it to 1024 x 768. The
resulting artefacts are clearlyvisible in the full-size image.
The is little value to this kindof interpolation, exceptpossibly to confuse thepurchaser. And, in fact quite afew cameras have beenretailed as having a certainresolution, when in fact it wasthis was an interpolatedresolution.
The amount of interpolation applied to the image is also important. In the exampleabove the image measurements of 640 and 480 are increased 1.6 times.
If in some cases interpolation can increase and even create artefacts in an image, itcan also be useful in some others. As mentioned earlier, the quality of theinterpolation depends greatly on the size increase it applies to the image.
Another example:
This photo of a printeddrawing was captured with a
Fujifilm S1 Pro, a camera thathas a CCD resolution of 3.4megapixel, and an interpolatedmode of 6.1 megapixel. It isworth noting here that thisparticular camera uses a largeSuper CCD which has adifferent pixel layout thanother CCDs. Super CCDs use adiagonal pixel layout thatyields more colour informationthan CCDs that have pixelsarranged in standard
horizontal and vertical rows.This in turn helps the precisionof the interpolation process,and indeed the Fujifilm S1'sinterpolation is one of thebetter ones.
Noise is a phenomenon encountered by all digital camera users at one time oranother. The term "noise" describes a type of interference to which a digital image issubject.
At its simplest level, a sensor counts photons striking its pixels as they reflect off a
subject. This is the "signal". However, one of the properties of a sensor is that notwo pixels are likely to "count" the same number of photons, even when exposed tothe same light source. That unpredictable variation in their counts is referred to asthe Background noise.
Aside from this Background noise, there are additional sources of noise that maketheir way into a digital image, such as Readout noise and Processing noise.These occur at each step of the process by which the signal created by the lightstriking a sensor is amplified and converted to a digital value. The noise is acombination of the random element that is part of the measurements with theaddition of the interference caused by the camera's electronics.
Even when there is no light striking the sensor, electrons accumulate gradually in itspixels. Called Dark noise, the signal it creates is indistinguishable from oneproduced by light.
For our purposes here, the origin of noise is only interesting in general terms. Moreimportant, is when noise can be expected and the ways by which it can beminimized.
With digital cameras noise tends to increase with 2 factors: CCD sensitivity and thelength of the exposure.
Noise is always present
A simple demonstration can beused to show the noise presentin any image sensor.
Here, a photo was taken withthe lens cap tightly coveringthe lens. After a 20 secondexposure, the image appearsto be completely black. Yet, if one half of the image is
selected in an image editorand that portion's contrast ispushed almost to themaximum, random pixels of colour become visible.
The origins of the noise visible in the upper frame encompass all the types of noisementioned earlier, including dark noise which has had sufficient time to accumulate.This is a simple test that can be done with any digital camera that has controllableshutter speeds.
It is important to note that if this process is repeated over succeeding frames, thenoise will vary from frame to frame highlighting its random nature.
Noise increases with sensor sensitivity (ISO)
The noise content of a digital image increases along with the sensiticity gain of the
The photos are captured usinga camera mounted on a tripod.
The image at left is reducedfrom the original 1600 x 1200pixel photo. The camera is setto 100 ISO and the exposuretime is 8 seconds. As theprocess of reducing the size of an image eliminates a lot of the finer details, a section of this photo, cropped to showthe upper branches of the treeand some of the sky, is shownat full scale below.
In the image above, the tree's outline is hard to discern, much as it would appear tothe eye prior to getting accustomed to a dark environment. Venus can be clearly beseen near the upper branches of the tree, and a few stars are faintly visible. Someblue dots, noise originating in the sensor, are also visible. One of these noisy pixels,a bluish dot, can be seen on the right of Venus.
The photo on the right issimilar, also taken with thecamera set to 100 ISO, but theexposure time has beenincreased to 16 seconds.
Note that the branches of the
tree now detach themselvesclearly from the sky and that athin high cloud has also beenimaged.
The section below is takenfrom the same place as theprevious one, and shows thatsection of the image at 100%.
The most obvious things about the image above are the clearly defined treebranches and, of course, the brighter sky and larger number of stars that can beseen. The intrinsic noise of the camera is also more visible. There are now quite afew bluish dots, including the one observed in the previous frame.
To make this more visible, a portion of the image above is presented below, butmagnified 300%.
As demonstrated, night photography is likely to include noise. The noise content canbe controlled to some extent by the length of the exposure, and by the sensitivity of the sensor: a lower ISO setting will means a slower response from the sensor and inturn a longer exposure; while a higher ISO setting will shorten exposure time.Deciding the best combination of these depends greatly on the specific cameraused, as no two sensors are totally alike. Therefore, the best way to ascertain thesettings that are the most noise prone — and therefore those which should beavoided — must be done by experimentation.
Finally, another factor should be taken into consideration. JPEG compression canaugment noise in an image by averaging the colours of groups of adjacent pixels. If available, an uncompressed image format should be used.
A date and time stampfeature is available onnumerous cameras, includingdigital ones. The system usedwith digital camera is roughlysimilar to the one used in filmcamera: the date and time are
placed directly into the imagerecorded by the camera,covering up the image in thatsection.
The time / date stamps comein varying sizes, dependent onthe camera manufacturer'staste, or the operating systemused in the camera.
Some can be"relatively"
small size,such as theone presentedhere, or trulycolossal,occupyingmore thanhalf the lowerportion of thewidth of theframe.
Actual size on a 1600 x 1200 photo.
While the time/date stamp can be useful: the need to identify the date and time aparticular image was recorded, it is also a permanent one, and something that canseriously impact the quality, or value, of a photo. In view of this fact, we shallexplore the other option that is part of the benefits offered by digital cameras.
The vast majority of digital cameras use an image file format called Exif applicable to both JPEG compressed and TIFF (uncompressed) images. The Exif (Exchangeable Image File) format is a JEIDA (Japan Electronic IndustryDevelopment Association) standard that was established in Oct. 1995, revised inNov. 1997 as version 2.0, and revised again in June 1998 as version 2.1; the formatcurrently in widespread use.
The interesting aspect of the Exif format is that it stores metadata right at the
beginning of the file. The Metadata can contain an enormous amount of informationabout the image. Not only will it record the date and time, it can also provideshooting data.
The table below is a partial listing of the metadata that can be encoded within theimage itself:
ExposureTime Exposure time (reciprocal of shutter speed) in second.
FNumber Actual f-number (f-stop) of the lens when the image was taken.
ExposureProgram Exposure program that the camera used when image was taken:
'1' means manual control,
'2' program normal,
'3' aperture priority,
'4' shutter priority,
'5' program creative (slow program),
'6' program action(high-speed program),
'7' portrait mode,
'8' landscape mode.
ISOSpeedRatings CCD sensitivity equivalent to film speed (ISO value).
ExifVersion Exif version number. If the picture is based on Exif V2.1, value is "0210".
DateTimeOriginal Date/Time of original image taken.Data format is "YYYY:MM:DD HH:MM:SS". If the clock has not set or cameradoesn't have clock, the field may be filled with spaces.
DateTimeDigitized Date/Time at which the image has been digitized.
ComponentsConfiguration Shows the order of pixel data.
CompressedBitsPerPixel The average compression ratio of JPEG (rough estimate).
ShutterSpeedValue Shutter speed by APEX value. To convert this value to ordinary 'ShutterSpeed'; calculate this value's power of 2, then its reciprocal. For example, if the ShutterSpeedValue is '4', shutter speed is 1/(24)=1/16 second.
ApertureValue The actual aperture value of lens when the image was taken. To convert thisvalue to ordinary f-number(f-stop), calculate the value's power of root 2(=1.4142). For example, if the ApertureValue is '5', f-number is 1.4142 5 =F5.6.
BrightnessValue Brightness of taken subject. To calculate Exposure(Ev) fromBrigtnessValue(Bv), you must add SensitivityValue(Sv).Ev=BV+Sv Sv=log2(ISOSpeedRating/3.125)ISO100:Sv=5, ISO200:Sv=6, ISO400:Sv=7, ISO125:Sv=5.32.
LightSource Light source, actually meaning white balance setting. '0' means unknown, '1'
daylight, '2' fluorescent, '3' tungsten, '10' flash, '17' standard light A, '18'standard light B, '19' standard light C, '20' D55, '21' D65, '22' D75, '255'other.
Flash '0' means flash did not fire, '1' flash fired, '5' flash fired but strobe return lightnot detected, '7' flash fired and strobe return light detected.
FocalLength Focal length of lens used to take image, in millimeters.
FlashPixVersion Stores FlashPix version. If the image data is based on FlashPix formar Ver.1.0,value is "0100". Since the type is 'undefined', there is no NULL(0x00) fortermination.
ColorSpace Defines Color Space.
ExifImageWidth
ExifImageHeight
Size of main image.
RelatedSoundFile If the camera can record audio data with image, shows name of audio data.
FocalPlaneXResolution
FocalPlaneYResolution
Pixel density at CCD's position. In a megapixel camera used to take a pictureat a lower resolution (e.g.VGA mode), this value is re-sampled by pictureresolution. In such case, FocalPlaneResolution is not same as CCD's actualresolution.
Below is a screen capture from one of the many programs (including some that areoften provided as part of the software that comes with a digital camera) which candecode the metadata encoded in Exif files.
In this case, note the highlighted area showing the time and date of the photo towhich the data belongs:
As can be seen here, since this information is available as part of a digital camera'simage files, it makes it possible to avoid permanently overwriting part of theactual image with the date and time.
Yet, the fact that the original image contains the data is only a partial solution.Often, photos need to be rotated, cropped, or altered for brightness, contrast,sharpness. If these images are then re-saved, the data is generally lost as fewimage editing programs, besides the newest ones, are able to retain the data withthe saved file.
In addition, some software provided by a few manufacturers transfers the imagesvia a direct camera -> computer link (serial or USB), into the memory (RAM) of thecomputer. Since in this case the files have not been directly copied to the hard disk,saving the photos from a non-Exif compliant application can permanently lose theshooting and date/time information. In this case, using a memory card reader,attached directly to the computer, allows copying the photos directly to the harddisk and safeguarding the metadata.
Still, the only true solution to these problems is to retain the original files from thecamera; and store them on CD-R or comparable support. Using a directory structureidentifying dates and or locations for each group of photo is the easiest.Alternatively, some image organizing programs make it possible to classify imagesby date, location, and subject. Any altered images can then be saved in anotherdirectory, leaving the original intact.
It is worth noting that a few manufacturers provide software with their cameras thatallow the image to be modified, all the while retaining its Exif format and themetadata. Fujifilm's Exif Viewer and Olympus' Camedia Master utilities, forexample, make it possible to alter certain aspects of an image such as orientation,brightness, contrast and—in the case of Camedia Master—distortion, and yet save
In conclusion, the Exif format used by digital cameras makes it possible to retainprecise image information including the date and time at which the photo wastaken, without permanently marring the image with a stamp. And all it requiresfrom the user is a bit of organization and some extra storage space.
An article on the various types of memory cards and storage has been available inour pages since April 2001. Now, in November of 2002, it's time to refresh it in lightof the changes that have taken place since then.
In this overview, we have separated memory cards — products that use flashmemory — from those evolved from disk and CD technology. Furthermore, we havealso removed the floppy disk and the Super Disk from this list. Floppy disks wereonce the staple of Sony Mavica cameras, but are now becoming rare, as arecameras that use Super Disks. Also worth noting, one newly announced memoryformat, Memory Stick Duo, is not listed here as it is not yet available andinformation about is still scarce at this time.
Finally, instead of providing a cost per megabyte which becomes inaccurate as timepasses and prices change, we have provided some links in order to allow genericsearching for the going prices for various types of card formats.*
To check prices of the various types of memory cards, simply click on the linksbelow their photo. Please remember that the search results will show prices in UScurrency.
* Note that due to an unavoidable delay, a couple of price search links will notbe available immediately. Once these links become available this page will beadjusted.
xD Picture Card (xD)
xD Picture cards are the newest type of memory card format. Jointly developedby Fujifilm and Olympus, it wasintroduced in the late summer of 2002.The xD picture card is the smallestformat to date, measuring 2 x 2.5 x0.17 cm (0.78 x 0.975 x 0.066 inches)and weighing 2 grams (0.07 oz).
Four types of cards will be availableinitially: 16MB, 32MB, 64MB and 128MB.256MB cards are planned to beintroduced in late 2002 or early 2003,with 512MB and 1GB to 8GB cards tofollow in 2004 and beyond.
To date, only Olympus and Fujifilm areproducing cameras that use xD cards.
Adapters and readers for the format are already available: a CompactFlash adapter(Olympus), a PCMCIA adapter (Olympus), and xD card readers from both Olympusand Fujifilm.
MutiMedia Card (MMC)
Price Search available soon
The MultiMedia card was originallydeveloped in 1997 by SanDisk andSiemens. The size of the proverbialpostage stamp, the card measures 2.4 x3.1 x 0.12 cm [0.94 x 1.2 x 0.046 inch])approx.
This card format currently offerscapacities up to 64MB. MultiMedia cardsare useable in a growing number of products, including cameras from
Kodak, Konica, Kyocera, Minolta andToshiba. MMC cards are generallyslightly less expensive than SecureDigital (SD) which was developed later(see below).
Secure Digital (SD)
A creation from Matsushita and Toshiba,Secure Digital cards look exactly like
MultiMedia cards, and cameras that usethem are usually compatible with bothformats.
Secure Digital cards have built-in dataencryption, one of the major differenceswith MMC, and also provide a slightlyfaster read and write speed.
Currently, the maximum capacity is256MB.
Price Search
CompactFlash (CF)
CompactFlash was the brainchild of SanDisk, the leading manufacturer of Flashmemory. Introduced in 1994, the cards have now become the most widely usedmemory format for digital cameras.
CompactFlash offers 2 slightly differentformats: Type I, and Type II. Type II is alittle bit thicker. Type II memory cardsreached capacities of up to 448MB, butappear to be losing ground to thethinner Type I form factor, Type I cardsare now available with a 1GB capacity.
IBM Microdrives however, use the TypeII format. (See further)
Type I and Type II cards measureapproximately 4.4 x 3.6 cm (1.78 x 1.4inch) and are 3mm (Type I), or 5mm(Type II) thick (0.11 and 0.2 inch).
SmartMedia (SM)
SmartMedia was developed by Toshibaback in 1995.
SmartMedia cards are very thin and aremore fragile than other formats. Theymeasure 3.6 x 4.5 cm (1.4 x 1.75 inch)and have a thickness of 0.5 mm (0.0195inch).
This format reached its maximumcapacity of 128MB back in 2000, andhas remained stagnant since. It is likely
that SmartMedia is on the way out, andthat manufacturers will turn to othersolutions.
Price Search
Memory Stick (MS)
Price Search
Memory Stick is a Sony invention.Introduced in 1998, Memory Stick isnow the dominant memory format forSony products. It is used widely in
digital cameras, computers, and the Cliéhandhelds. Memory Stick is also startingto appear in the product of othermanufacturers, most notably Konica.
Memory Sticks are long and narrow,measuring 2.1 x 5 x 0.25 cm (0.82 x1.95 x 0.0975 inch). Although a 256MBcapacity was announced in late 2001, itremains unavailable at this time.
The creation of IBM, the Microdrivestarted with capacities of 170MB and340MB when introduced in 1998. Theyquickly graduated to 512MB soon after,and reached the 1GB mark in 2000.Microdrive support is now offered on awide variety of cameras from many
manufacturers: Canon, Casio, FujiFilm,Kodak Professional, Kyocera, Minolta,Nikon and Sanyo.
Although not as resistant to impact asCompactFlash, Microdrives have provento be reliable, solid, and one of the mosteconomical storage devices available fordigital cameras.
Mini CD-R, Mini CD-RW
Evolved from the regular size CD-R andCD-RW discs, 8 cm discs have been usedsuccessfully in a number of — mostly Sony— cameras.
CD-R are very economical, and although abit more expensive, so are CD-RW discssince they can be re-written, albeit at aslower speed. The downside is that writingimages to a CD is slower than to flash
memory, or a hard disk, and that themedium tends to slow down the camera.
Still, the format offers the advantage of being recorded on a medium that is verybroadly supported, and easily read with acomputer. Price search unavailable
For many people considering the purchase of a digital camera for the first time, onequestion that keeps cropping up is "how many megapixels to buy?"
One way to answer the question is to work on the basis of the finished print size.The number of pixels required to print an image so that the print becomes almostindistinguishable from a print made from film, varies according to the size of theprint. Indeed, past a certain number of pixels (around 1.3 megapixels), the quality of the image is not directly linked to the number of pixels on the sensor, the imagecontains a sufficient amount of information to create a clear and sharp photo.
Digital images can be printed in different sizes, but to get the best quality image,
printers need a sufficient number of pixels every square inch, or square centimetres,to produce a print that appears smooth. In short, the number of dots placed on thepaper must be sufficiently high that the eye will not detect jaggies, or otherartefacts.
The vast majority of digital images, in particular JPEG format images, start life witha pixel per inch (PPI) count of 72. The reason for this is that the image is destined
for a monitor and that 72 pixels per inch is the standard definition of the latter.
When this count of the image's Pixel Per Inch is converted to the Dot Per Inchcount of a printer, the image's size changes accordingly as the printer requires manymore dots per square inch than the monitor's 72 pixels per inch.
Probably the easiest way to visualize this is by looking through the chart below. Thechart assumes a printer DPI (Dot Per Inch) of 300, which will usually yield a sharpimage without any obvious artefacts:
Camera Screen image Printed image
Image Resolution = Image size at 72 PPI
(pixels per inch)
= Image size at 300 DPI
(dots per inch)
A 640 x 480 =22.58 cm x 16.93 cm(8.889 in. x 6.667 in.) =
5.42 cm x 4.06 cm(2.133 in. x 1.6 in.)
B 800 x 600 =28.22 cm x 21.17 cm
(11.111 in. x 8.333 in.)=
6.77 cm x 5.08 cm(2.667 in. x 2 in.)
C 1024 x 768 =36.12 cm x 27.09 cm
(14.222 in. x 10.667 in.)=
8.67 cm x 6.5 cm(3.413 in. x 2.56 in.)
D1280 x 960
(1.3 megapixel)=
45.16 cm x 33.87 cm(17.778 in. x 13.333 in.)
=10.84 cm x 8.13 cm(4.267 in. x 3.2 in.)
E
1600 x 1200
(2.1 megapixel) =
56.44 cm x 42.33 cm
(22.22 in. x 16.665 in.) =
13.55 cm x 10.16 cm
(5.333 in. x 4 in.)
F1800 x 1200
(2.3 megapixel)=
63.5 cm x 42.33 cm(25 in. x 16.665 in.)
=15.24 cm x 10.16 cm
(6 in. x 4 in.)
G2048 x 1536(3 megapixel)
=72.25 cm x 54.19 cm
(28.444 in. x 21.333 in.)=
17.34 cm x 13 cm(6.827 in. x 5.12 in.)
H2400 x 1600(4 megapixel)
=84.67 cm x 56.44 cm
(33.333 in. x 22.22 in.)=
20.32 cm x 13.55 cm(8 in. x 5.33 in.)
As can be seen by looking through the dimensions of the printed photo produced byvarious sensors, it is clear that the larger the final print, the more pixels the sensorwill need.
Most mini-labs produce printed photos from 35mm film that measure around 6 x 4inches or 15.24 cm x 10.16 cm. To obtain a good print with a similar dimensionfrom a colour printer, the camera must be able to record an image of at least 2.16megapixel.
Although not to scale, the image below shows the relationship of size between a 640x 480 resolution (A) and a 4 megapixel resolution (H) representing a print size of 8x 5.33 in or 20.3 x 13.5 cm.
A note of caution is in order: colour printers, or the software driver that they useare able to interpolate an image to larger dimensions. Interpolation involves the useof algorithms to "invent" extra pixels which are inserted between existing ones inthe image to increase the overall size of the image. This is a process that mostprinters use to smooth the appearance of the image. However, if the interpolation istoo great — the image size is increased too much — then artefacts will appear in theprint.
Finally, many photo printing programs perform the changes from 72 DPI to a PPIvalue automatically, adjusting the parameters to the image size requested by theuser. However, to get the best results, it is advisable to avoid printing a lowresolution image at too large a size.
One question crops up regularly in the e-mail we receive at megapixel.net: which isthe best format to use when taking photos? When is an uncompressed formatappropriate? And if acompression is used, what level of compression should beused?
Many manufacturers preselect a compression level for their cameras, making it thedefault. Often, their selection appears to be based on the need to strike a balance
between the storage space available (with the memory card that comes with thecamera), and the need to capture a sufficiently high enough quality image that theuser won't be disappointed by the results.
Obviously, the crux of the decision relies on a fairly subjective factor: one's
perception of what constitutes an acceptable image quality. To some, a few artefactsare acceptable; to others, none should be detectable. The trade-off is the space theimage takes up on the memory card.
Of late, the compression algorithms available on digital cameras have greatlyimproved in quality. Recently, with the cameras from some manufacturers, thehighest image quality JPEG compression has become very difficult to differentiate
from an uncompressed image; even when the image is examined undermagnification. The result has been that for most purposes, the highest qualitycompressed format is reliable and presents no problems when the image is printed.Yet, the availability of an uncompressed format on a camera remains an advantagefor some uses.
JPEG is a lossy image compression. This means that when the image is compressed,part of the data captured by the sensor is permanently lost — irretrievably lost. (Tounderstand how the process of compression works see our article on Compression)
The use of an uncompressed format involves some important side effects. Thecamera takes considerably longer to store the image — in the case of some 4 and 5megapixel cameras, the storage time can be upwards of 30 seconds — and the filesize requires the use of high capacity memory cards. While memory card priceshave been dropping, the expense involved in the purchase of additional memorycapacity can significantly add to the overall cost of using a digital camera.
Therein lies the dilemma: what is the best format to use? Our answer to thatquestion is conservative. Since most of the time the photographer doesn't knowwhat the ultimate use of the image will be, it is safer to err on the side of caution,or in this case, of quality. This means using the highest quality JPEG compression asthe "normal" setting when taking photos. The setting will normally avoid the worsttype of artefacts, such as blotches that can appear in a blue sky, or fuzziness on theedges of objects.
The level of compression used becomes critical when the image is used at its full-size. Should it be reduced, then the decrease in size effectively reduces, or eveneliminates, most artefacts.
To illustrate the process, we used a Canon S40. Canon, along with other top cameramanufacturers, have JPEG formats that provide a very high image quality, evenwhen the compression is quite strong.
Our subject is a section of anantique Indian painting,photographed in macro mode.
The same photo is captured at3 increasing levels of compression, and once usingCanon's proprietary RAWformat, which employs a non-lossy compression to store theimage. The RAW image is thenconverted to a standard TIFFformat with Canon's software.
With enough magnification,the effect of the JPEG processis detectable at all levels of compression.
To illustrate this, all the sections shown below are screen captures made with theimages magnified to 300%. The images below are cropped to show the tip of thispretty lady's nose, and a bit of the green background.
As can be seen in these images, a checkerboard pattern becomes progressivelymore discernible as the compression is increased, and is absent in the RAW formatimage. The compression is most observable in the organized square patterns thatappear in the green area, and on the tip of the nose.
We want to emphasize that the cropped images above are shown greatly enlarged .
To the naked eye, and seen at 100% scale, the effect of the compression is barelydiscernible.
The level of compression applied to the image becomes important if and when theimage is printed. The least amount of compression will usually produce a betterprint, but much depends on the subject.
If the image is going to be printed in a publication such as a newspaper ormagazine, or printed with a dye sublimation photo printer, the least amount of compression, or an uncompressed format, is preferable. If the image is going to beprinted on an ink jet printer, or simply used electronically, then the JPEGcompression can be set to the best quality or a medium level. If the image's size is
going to be drastically reduced — such as would be the case for an image to be sentby e-mail to a friend or relative, then a strong compression can be used. However,in all cases the decision is final. The image will never be better than what thecamera saved.
The finality of that decision is the one factor that has always convinced us to use aconservative approach. Once the data has been subtracted to reduce the file size, itis gone forever. And in our view, this one simple fact should dictate the format inwhich the images are saved. In short, better use the highest quality possible, investin extra storage, and alter the size, or compression, of a copy of the image, whileretaining the original.
Cameras tend to dust and fingerprint magnets, and cleaning them without causing
some accidental damage can be a bit worrisome. Yet, a good cleaning can be donewith some simple supplies and, if done carefully, will help keep a camera lookingnew for years.
Many cameras come with little stickers on them. Witness the number of camerasthat can be seen, years old, still sporting their little oval "OK Passed" sticker, its goldcolour faded and its edges peeling. Many people avoid removing these thingsbecause they worry that the attempt will damage the finish of the camera. In fact,leaving those stickers on is often a mistake. As time passes, many surfaces exposedto light change colour slightly — even metallic ones — and the adhesive of thesticker may well become permanently etched into the finish. Moreover, they have novalue as they simply indicate that the camera was quality inspected after assembly.
Removing these and others like it doesn't invalidate a warrantee, nor compromisethe resale value of the camera.
To clean a camera properly, one has to adapt the products and methods used to thespecific camera. Different surface finishes require different handling. For example, aplain plastic with a silver finish isn't very strong and any cleaning of the surfaceshould be done with great care to avoid rubbing off the finish; on the other hand, ametal finish is usually quite rugged and can be cleaned easier.
This is why prior to any first-time cleaning, it isimportant to determine with some degree of certainty the composition of the surface. Mostcamera bodies have some sort of compartmentwhich, when opened, can be used to determine thefinish of some the camera's surface.
A good place to look in is the battery compartment.Often the cover will be made of plastic, but the bodymay not be. A look inside the cavity for the batterycan reveal the type of finish used. In the case of thephoto at left, the camera has a silver finish on theoutside, but a look inside reveals a light grey plastic.This is a surface where the exterior finish is like apaint, and therefore a fragile surface.
The same thing can be done to determine if thecamera has a metal body and the type of finish ithas. In the photo at right, when the batterycompartment is opened, the metal casing of thecamera is visible (1), next to the plastic inner body(2), and so it the fact that in this case, the metalsurfacing has a paint finish.
Once it has been determined how the body isfinished, cleaning it becomes a lot safer.
Whether the lens has a large front element,or a small one, the first step is to blow someair on its surface to remove anything thatmight scratch it when it is cleaned. Maintainthe compressed air can upright, and shootsmall burst of air at an angle from the lens.Do not use long bursts and do not bring the
tip of the nozzle closer than 3 inches.
Use a cotton cloth to wipe the lens, whileexhaling directly on the glass. For the areasthat the cloth does not reach, use a cleancotton swab to gently dislodge the dust. Acotton swab must be used extremely gently,and never press on the tip, always use it
at an angle, and never use a chemical.
For lenses with a larger front element, acotton cloth works best. Here again, afterspraying a burst of compressed air at the
lens, simply exhale lightly on the glass andwipe with a good quality cotton cloth. Thisshould remove most stains. Water rings, fromrain or other moisture, can be quite stubborn.Any water droplets should be dried right awayand not allowed to dry on the glass.
Afterwards, some compressed air should beable to blow any remaining dust and lintaway.
Cleaning the viewfinder:
Because it is so close to the eye when the camera is in use,dust in the viewfinder can be quite noticeable, and causeblurred areas in the image. For some cameras, theviewfinder can also be the most difficult to clean becausethe space is so small.
The most difficult viewfinders to clean are those that aredeeply recessed, when the lens of the exit pupil is far insidethe body.
The glass, or optical grade plastic, of most
viewfinders have no outside coating, and therefore itis safe to use a mix of 50% glass cleaner and 50%water to clean them. Touch the extreme tip of acotton swab to the mixture. The tip must be damp,and not soaked. If it is too wet, squeeze out theexcess before gently touching it to the surface of theviewfinder. Use a gentle side to side motion withoutapplying any pressure. Then, immediately use thedry end of the swab to gently wipe the glass orplastic. Do not allow the cleaned surface to air dry.
Any dirt or dust remaining in the corners can also be removed. Use a new cottonswab dampened slightly with the same mixture on the tip and let it sit in the corner
for about 10 to 15 seconds. Then dry the area with a fresh swab and usecompressed air to blow away any remaining dust or lint.
The rubber used for some parts such as the grip, orthe viewfinder eyecup, attracts lint, dust etc. Toclean these parts, use Isopropyl alcohol. Dip a cottonswab in the alcohol and squeeze its tip lightlybetween the fingers to wring out excess alcohol.
Using the cotton swab, wipe the eyecup or grip
evenly. Do not use an excessive amount of Isopropylalcohol or the drying alcohol will leave streaks. Avoidgoing to close to parts — such as a painted andtextured body surface — that might be stained ordamaged by the alcohol. As soon as the rubber partis wet with the alcohol, dry it with compressed air.
Cleaning the LCD screen:
There are different types of surfaces forLCD screens. Some are non-reflective
and lightly textured, some have a shinyprotective glass or plastic. Whatever thecase, the LCD screen is fragile. Anystrong pressure on its surface candamage it.
The LCD screen should be cleaned theway a lens is: with a soft lint-free cloth.Exhale on the surface lightly and wipeaway fingerprints or nose prints. Forstains that don't come off with just acloth, use a cotton cloth moistened with
a mix of 50% glass cleaner and 50%water. The cloth should be damp to thetouch, not soaked. Gently moisten thestain, then wipe with a clean dry cloth.
Memory card compartment:
Never insert anything other than a card into the memory card compartment. Thereare small and very fragile contacts inside that can be damaged easily.
If cleaning is required — for example the
camera has been used in a very dustyenvironment — the best way to clean thecard slot is with compressed air.
When using compressed air or gas, the samerule always applies: do not insert the nozzleinto the card slot; and do not spray from tooclose a distance or otherwise moisture willget into the camera. Maintain at least acouple of inches between the tip of the sprayand the card slot; and clean dust out usingshort, quick bursts.
Battery contact can become soiled over time and affectthe performance of the batteries. The common way of cleaning these is with the small eraser at the end of apencil. The eraser will remove most dirt and even somelight corrosion. Once the contacts are cleaned, thenblow air into the compartment to remove the remainingbit of eraser and dirt.
A word of caution:
The cleaning methods presented here are based on experience and should workwith most cameras. However, if you decide to clean your camera yourself, becareful, work slowly and gently as ultimately, you are responsible for your actions.Remember that such things as cleaning solutions and alcohol are solvents and if used inappropriately or carelessly can damage the camera.
If you are unsure what to do, have the camera cleaned by a technician. It isbetter to spend a bit of money to have your camera cleaned than to damage it.
The name ISO has its roots in the Greek word "isos" meaning "equal ". The name of the organisation that sets these standards, commonly referred to ISO, is in fact theInternational Organization for Standardization* and not an acronym for the ISOdesignation. The ISO standards have replaced the previous ASA (AmericanStandards Association) ones, which used to establish North American standards forfilm manufacturers.
Not surprisingly, the ISO rating given for digital cameras has its roots in filmphotography. As with film photography, the ISO is a measurement of the sensor'ssensitivity to light. Low numbers such as 50 ISO, 64, 100 ISO indicate a sensorthat reacts less quickly when exposed to light than higher numbers such as 200ISO, 400 ISO.
Much like film, image sensors used in digital cameras are "set" to emulate filmspeeds. And, again much like film, the higher the ISO speed, the more noticeablethe "grain" becomes in the image. However, the grain one can see in a digital imagetaken at a high ISO is really interference or noise, and not film grain — beads of chemicals on the emulsion — that can be observed in high ISO film images.
The most important difference between image sensors and film is that thesensitivity of the sensor can be changed, while with film, the film has to bechanged. In fact most digital cameras provide the user with a ISO setting thatmakes it possible to increase the sensitivity of the CCD, and therefore allow the use
of higher shutter speeds and/or smaller apertures according to circumstances.
A look through the images below shows the influence of the ISO setting on theshutter speed.
The photos are taken in subdued light, and with the camera (in this case a MinoltaDimage S404) mounted to a tripod.
By increasing the ISO setting, the shutter speed goes from 1/8 of a second — adifficult shot to take without a tripod to stabilize the camera — to 1/45 of a second,a shot that could have been made without a tripod. The small sections of the fullsize images that are included show an area susceptible to a gain in noise, whichmakes the image "more grainy". As can be seen, the increase noise is quiteacceptable and the benefit of a higher shutter speed can be worth it in somesituations.
A sensitivity of 400 ISO isn't the limit. Some cameras are able to range higher, up to
800 and 1600 ISO. The most common additional sensitivity setting is 800 ISO, andalthough the noise can become more pronounced, recent models have shownconsiderable improvements in noise reduction over what earlier models could do at800 ISO. The two images of the same subject below show these results.
As can be seen, the noise increase at high ISO settings is noticeable but, once
again, not unacceptable. Furthermore, a 1600 ISO sensitivity permits trading someof the shutter speed for a smaller aperture, which produces a greater depth of field,as in the example below:
Sensitivity: 1600 ISO
Aperture: f13
Shutter Speed: 1/15
The effect of the smaller aperture (f13) can be seen in the greater detail that shows
in the wooden mallet, part of this Tibetan singing bowl.
Auto ISO Versus Selectable ISO
There are many different digital cameras on the market, but when it comes tosensor sensitivity, they can be grouped into 3 categories:
1: cameras that have a single, non-variable ISO sensitivity,
2: cameras that automatically vary the sensitivity of the sensor accordingto the ambient light and have no user setting,
3: cameras that offer both an Auto setting, and user-adjustable ISOsettings.
For cameras in our category 1 above, the advantage is simplicity, and in some casesa lower purchase cost. The disadvantage is that the camera may often require the
assistance of its flash to get a properly exposed image. Furthermore, it may beforced to use low shutter speeds when the weather is poor and there isn't sufficientlight.
For cameras in category 2, the advantage is a greater flexibility for taking pictures,but the disadvantage is that the image quality may lack consistency. Some camerasare quite prone to noise, and the image quality can vary considerably when the
camera uses a low ISO setting and when it uses a high setting. Ideally, an Auto ISOsystem should have a clearly defined range that avoids the noisier settings, and infact, many cameras have limits on the range the Auto ISO is allowed, either 50 to100, or 100 to 200 and in some case 100 to 320 ISO.
The last of our camera categories, 3, is the most flexible. These cameras allow theuser to choose a particular setting. For example, this makes it possible to increasethe sensitivity to take photos in a museum without the flash, or, outdoors light toobtain a greater depth of field through the use of a smaller aperture.
As with many other things, in our opinion, having a choice is always better thanhaving none.
The Exif Print logo is becoming common on digital camera boxes, and printers.But, unless you follow every development of the digital imaging industry, the oddsare you may not be acquainted with what it stands for, and the importance of Exif,
in general, for digital camera owners.
Exif is an acronym for Exchangeable image f ile, and is the format used for digitalcamera photos. The format establishes the specifications for optional attributeinformation, as well as stipulations relating to format implementation. Moreover, italso records what is commonly called the shooting data — the settings in use by thecamera when the photo was taken — and specifies the way the image should beformatted so that it can be used by Exif compliant devices such as printers.
The Exif format was developed back in 1996. At that time, the Japan Electronics andInformation Technology Industries Association, JEITA, recognized a need toestablish standards for the digital camera manufacturers. The result of these effortswas the creation of the first standard Exif 1.0 which was released in late 1996.
By the middle of 1998, digital cameraswere starting to become more popular andJEITA upgraded the standard with therelease of Exif 2.1, a revision that addedfurther specifications including some forincorporated audio files, allowed for morecomplex images, changed the
chrominance sampling method, andestablished the way thumbnails would beincluded in the image header.
Exif 2.1 became widely adopted, and forthe consumer, the result was the superbamount of details available within eachand every photo.
An excellent example of the volume of information contained in the Exif formatcan be seen with the use of a programsuch as Exif Reader. In the screencapture shown on the right, all the areashighlighted in gray relate directly tocamera settings at the time the photo wastaken. In addition, numerous otherparameters are stored, and in the case of this particular image recorded with aCanon EOS D60, includes all the settingsfor the Custom Functions (of which thereare 14), and the orientation of thecamera.
This screen capture is of a shareware program calledExif Reader Version 2.70 written by Ryuuji Yoshimoto.It can be downloaded from his site, or by clicking onthe image at right.
In April of 2002, JEITA released its latest addition to the Exif standards, an extendedformat designed to include parameters useful for printing photos.
JEITA's White Paper describing the additions states that:
"printers usually analyse image data and process the data based on general assumptions about the best processing method. This method usually produces good
printouts, but sometimes depending on conditions the processing carried out is not the best for that image."
In other words, it would seem that the generic process used to print photos couldlead to some poor quality prints; and that were the printer given additionalinformation about the camera settings, and the conditions at the time the photo wascaptured, a better image would result.
Indeed, this was an observation that had already been made by somemanufacturers — Epson comes to mind — and some had already implemented PrintImage Matching (PIM) technologies into their products. So, in a way, Exif 2.2represents a standardization to implement PIM across the entire industry, both forcamera manufacturers, the "writers" of the image files, and "readers" that wouldproduce equipment to print the images.
That standardization is the reason for which Exif 2.2 is also known as Exif Print. Itsextensions are specifically designed to supply Exif 2.2 compliant printers with the
set to automatic or manual during shooting. The tag should make it possibleto determine whether a white balance correction process should beimplemented at the printing stage. In general, printers automaticallyimplement the colour balance collection if the value is set to "auto whitebalance". Printers do not implement the process if the value is set to"manual." This feature should make it easier for the printed image to befaithful to the user's intention at the time of shooting.
Type of Scene:
Landscape: contrast, saturation, and sharpness are enhanced.
Portrait: memory colour correction of skin colour.
Night Scene: inhibition of soft colour tones, noise reduction.
Clearly, all this information can be used to improve the printed image substantially.Able to read that information, the printer no longer prints a night scene the way itwould process a landscape, which is what most printers did previously since thesame processing technique would be applied to every single image.
Exif 2.2 — Exif Print — adds another link in the chain that makes completely digitalphotography possible by applying the same type of correction that a person making
prints from film would use.
The result of the implementation of the Exif 2.2 standard should ensure that aprinted photo should look the same as the original.
* Most of the information presented here originates from white papers available on theJEITA web site.
The goal of this article is to provide an overview of some of the most commonlyencountered optical distortions. Distortion is often mentioned — either as beingpresent to some degree, or absent — when compact digital cameras are considered,because of the very short focal lengths of their lenses. We have no intention of getting into any technical detail here. Optics is a highly complex field, and not ourarea of expertise. The aim here is simply to explain the most important types of distortion as they relate to compact digital cameras, and the methods employed tocorrect them and minimize their effect on images.
Focusing light, and the distortions thatthe process can induce wasrecognized and understood longbefore the development of photography. The need to focus thelight from lighthouses for example,caused Augustin Fresnel to create anoptical surface — one that still bearshis name to this day, the Fresnel lens— to focus light over a long distance.Just one of many lens types invented,
and part of the history of opticalscience.The Fresnel lens of an old lighthouse in the Halifax
Maritime museum.
In the context of photography, references to optical distortion apply to specific, and
undesirable, anomalies that occur in the process of focusing light onto a surface.Over the years, complex lens designs and optical formulas have been devised toeliminate, or minimize the anomalies.
Distortions occur as light passes through the different glass elements that make upa lens and which focus the image. The function of the lens is to capture all lightrays, at all the visible wavelengths, and to precisely place them onto a small flat
area while maintaining a sharp image. Since light travels on a straight path, theoptical elements of the lens must bend, and correct all the light reflected from ascene. For instance, a subject measuring 3 meters across must be sharply focusedon a surface less than 2 centimetres wide in the camera.
As light travels through any of theelements of a lens, as it enters and exitseach element, the curved surfaces changeits angle, bending it as required.
To allow the light to pass throughunimpeded, not only must the glass becompletely free of any impurities which
might disperse it, it also has to minimizediffraction which might cause a loss of focus as it reaches the image plane.1. Converging lens. 2. transmitted light.
3. Focal point. 4. Image plane.
With zoom lenses, coatings and the complex geometry of each element are in largepart responsible for the cost associated with the construction of the lens. Zoomlenses must control and focus the various wavelengths of light, limit dispersionthrough the use of specially formulated glass (ED glass), while simultaneouslycorrecting most of the distortions that are induced by the various possible focallengths of the zoom.
Certain types of corrections though, are more difficult to obtain than others, and ithappens that lenses that are otherwise excellent, suffer from slight distortions atthe periphery of the image plane, where light arrives at a more pronounced anglethan at the centre of the image plane.
With compact digital cameras, the small size of the sensor requires the use of muchshorter focal lengths than 35mm cameras. This in turn makes the neededcorrections even more complicated.
Barrel distortion
One of the most commonly observed types of distortion with a wide angle is barrel distortion. As the name implies, the image is distorted into a barrel shape, curvingoutwards at the edges.
While barrel distortion is a desirablefeature for one type of lens - called afisheye, which allows for creativeimages - with most wide anglelenses, the lens should form animage that looks as natural aspossible. To this end, barrel
distortion is corrected as much aspossible.
To control barrel distortion,aspherical surfaces are used onsome of the elements of the lens.Aspherical surfaces provide a way toapply a gradual correction thatprogressively increases towards theperiphery of the lens, and whichstraightens out the image.
The barrel distortion shown here has been exaggerated toillustrate the effect.
Pincushion distortion
The pincushion distortion shown here has been exaggerated toillustrate the effect.
If barrel distortion can affect thewide angle end of a lens,
pincushion distortion is itsequivalent at the telephoto end.
Pincushion distortion can show upin zoom lenses that have a broadrange of focal lengths. For example
a 10X zoom that has a focal lengththe equivalent of a 40mm to a 400mm.
Pincushion distortion causes theouter parts of the image to curveinwards, towards the centre.
Astigmatism
Astigmatism is a not a distortion per se, but a phenomenon that causes the outercorners of an image to loose sharpness.
Generally, it is caused by anirregularity, or an inappropriatecurvature in the outer part of the lens,causing a loss of focus when the lightreaches the peripheral parts of theimage plane.
This anomaly can be sometimes seenwhen a zoom is at its widest angle, apoint at which the greatest surfacearea of the lens is used, and when theproximity of the combined opticalelements can engender a loss of sharpness on the peripheral part of the image. It is also a phenomenon
that can be seen with some simplefixed-focus lenses.
Chromatic aberrations
In the area of phenomena that can cause a loss of image quality, chromatic
aberrations are variations that are not caused by the geometric qualities of the lenselements, but by the way they let light pass through the glass. The optical surfacesof optical glass are usually treated with special coatings that control the way light istransmitted.
As each colour component of light hasa specific wavelength, the coatingsserve to control them so they arrive atthe same point on the focal plane.
In some cases however, this controlisn't perfect. Then, all the colour
components of light are not preciselyfocused where they should be. Theresult is a purple, or violet fringe onthe edges of image elements.
In these cases, the aberration is mostvisible in the periphery of the framewhere the incidence of the light raysis the most pronounced, and atlight/dark boundaries within theimage.
Vignetting
Vignetting is a loss of brightness in the corners of the image. In this case the sourceof the problem is most commonly an external element to the lens, due to somethingblocking some of the light from entering the lens.
The problem can be caused by apoorly designed lens barrel or lenstrim, but is most commonly the resultof an inappropriate lens hood, or theuse of a filter not large enough tocover the focal range of the lens.
Many manufacturers avoid thisproblem by engineering hoods specificto the zoom ( perfect lens hoods)which have a "flower" shape thatoffers protection for the lens fromsunlight, but which is adapted to allthe lens' focal lengths.
We have covered here those phenomena that are the most frequently mentioned inour reviews. Quite obviously, correcting any of these demands high quality lenses,
which adds to the price of a camera. Aspherical surfaces or ED glass for instance,entail a greater cost when lenses are designed and built. Yet, it is a necessary costas it directly impacts the quality of the image, and needs to be valued as much asthe sensor itself.
With this article, we cover a subject that is critical to digital imaging: the quality of
the image reproduction on a computer.
Most digital cameras today capture excellent images, but to be appreciated, or
edited accurately, a good computer display of the photo is required. The way adigital photo looks on a monitor is dependent on a number of factors beyond theimage quality of the camera itself, or the talent of the photographer. Two basiccomponents of a computer are involved in determining its ability to display a naturalcolour image:
the capabilities of the video card and its settings,
the capabilities of the monitor and its settings.
These 2 elements decide how an image will be displayed; and ideally, theircapabilities should complement each other.
The video card
The video card is responsible for "translating" the computer's binary information intoa format that the monitor can use. Although this sounds a bit technical, the basicrequirements are quite straightforward and can be summed up in two specificcapabilities: the refresh rate and resolution, which are linked, and the colour depththe video card can display.
For the video card this means that it must have a fast video processor, and as muchmemory as possible. The quantity of memory (video RAM) controls the number of colours the card is be able to generate and the processor the speed at which it
redraws the image on the screen for any given size image [resolution]. The colourcapability of the video card is referred to as the colour depth, and the refresh ratesare quantified in Hertz. To get the best image possible from a video card it needs tobe capable of fast refresh rates and have enough onboard memory to display 24, or32 bit colour, referred to as True Colour.
Of these two things, only one needs to match the capability of a standard monitor:the refresh rate/resolution combination. The refresh rate — the number of timesthe image is redrawn on the screen per second — is critical to obtain a flicker-freeimage. The human eye detects refresh rates below 70 Hertz quite easily, and refreshrates below that threshold will cause significant eye strain over long periods. 70Hertz must always be considered the lowest acceptable refresh rate, and
nowadays many video cards and monitors exceed this commonly.
To get the highest numbers of colours to display on a monitor at high resolution, thevideo card must generally have at least 16MB of RAM onboard. However, even morememory (32 MB and up) will allow the card further capabilities, particularly withprograms that use 3D.
The differences between colour depths
Lets start with a simple fact: no display can show a photo appropriately if it showinganything less than 16 bit colour (High Colour). 16-bit colour is the absoluteminimum requirement. Using 16-bit, a video card has a colour palette of 65,536
colours to work from. While this sounds like a lot, the human eye can perceive manymore distinct shades than that; and a digital image can contain much more colourinformation than that. Below that, the is an 8-bit display, common a few years back,which can only produce a palette of 256 colours, acceptable for business graphicsbut not photos.
The Dot Pitch of a monitor describe the physical distance between similar colourpixels from two adjacent triads. (A triad is the term used to describe each group of 3 different colour pixels: red, green and blue).
The way the dot pitch is measured is dependent on the type of picture tube used in
the monitor. A traditional monitor uses a Cathode Ray Tube (CRT) with a shadowmask. The shadow mask is a metal screen perforated with minute holes throughwhich the three electron beams pass, allowing them to strike a single point on thephosphors lining the internal surface of the tube.
Another type of tube is the Trinitron picture tube, developed by Sony. These typesof tubes use an aperture grille instead of a shadow mask. The aperture grilleconsists of extremely thin vertical wires which take the place of the shadow mask.The pixels on these monitors are rectangular, not round.
For monitors that use anaperture grille the dot pitchis measured horizontally
as the distance betweentwo phosphors of the samecolour.
With monitors that use ashadow mask, the distanceis measured diagonallyfrom one colour pixel tothe next pixel of the samecolour in the next triad.
Generally the smaller the
distance, (the smaller thedot pitch number) thecrisper the image themonitor will produce.However, the dot pitchfrom an aperture grillemonitor cannot becompared with the dotpitch of a monitor using ashadow mask. Onlymonitors using similarsystems can be compared.
Resolution and Refresh Rate
In addition to having the best (smallest) dot pitch possible, the monitor must alsobe able to produce a high resolution image with a refresh rate that avoids flicker.The resolution and the refresh rate of a monitor are linked because, broadlyspeaking, as the resolution increases the refresh rate drops. This is why these twofactors are generally considered together.
Let's define the terms. Resolution describes the number of pixels that can bedisplayed, both horizontally and vertically on the monitor. The original resolutionemployed for monitors was VGA which is 640 x 480 pixels. Over time, theresolutions available to monitors have increased. Common resolutions nowadays are800 x 600; 1024 x 768; 1280 x 1024; 1600 x 1200; and on very large monitors upto 2048 x 1536.
To use a high resolution, the monitor needs to have a sufficient viewable area, and
usually this means that on a 17 inch monitor — the de facto standard today — aresolution of 1280 x 1024 is the maximum practical resolution, as above that, textbecomes hard too difficult to decipher for most people. With a 19 inch monitor, 1600x 1200 is the next step, and with a 22 inch monitor a resolution of 2048 x 1536 ispossible. Of course, as the viewable area gets bigger, the monitor gets moreexpensive.
The refresh rate is the vertical frequency, or the rate at which each horizontal rowof pixels on a screen is redrawn. A low refresh rate causes a visible flicker in theimage; a high refresh rate generates a clear and stable image. The standard forflicker-free images has been officially set to 85Hz. Nevertheless, most people don'tdetect flicker until the refresh frequency gets below 70 Hertz.
Of course, the video card must support the same combinations of resolutions andrefresh rates as the monitor. For example, if the monitor is capable of a resolution of 1280 x 1024 at 75 Hertz, then the video card must be able to produce it. In fact,when purchasing either a monitor or a video card, this compatibility must be takeninto account to make an appropriate selection.
Windows Settings
The majority of personal computers use Microsoft's Windows environment, and thisis the environment that we will focus on here.
On a Windows computer the settings for thedisplay can be accessed in either one of twoways. Right clicking on the screen (not in awindow) calls up a little menu with"Properties" as one option. Selecting"Properties" brings up the Display
Properties window (at left).
Alternatively, selecting the Control Panel fromthe Task bar (Start - Settings - Control Panel )
also provide an access to the Displaysection which opens the Display Properties.
The Settings tab (circled) of the DisplayProperties is controls the settings of the videocard and monitor.
The Settings window permits adjustment tothese 2 basic properties of the computer'sdisplay:
1. the colour depth,2. the screen area (resolution)
The Advanced button (circled in red) is theaccess to parameters such as refresh ratesand display font sizes. The contents of thissection will depend on whether the video cardhas its own specific driver, or if the Windowsdefault drivers are used.
(Note: reference must be made to the usermanual of the monitor to set theseparameters since the monitor must supportthe combination selected.)
Windows provides definitions for many brandsof monitors, besides generic descriptions whichcan help ensure that the refresh rates areappropriate for the monitor.
Other Setting of the monitor
Most current and recent monitors provide adjustments for some of the monitor'sown settings such as contrast, brightness, and image size. With most CRT is ispreferable to keep the brightness setting down and the contrast setting up,asincreasing the brightness too much will produce washed-out colours and shorten thelife of the picture tube. For the Contrast , increasing it so that black is black (notgrey), and white is white (but not blinding) will help all the other colours displaybetter.
Many monitors also include controls over the geometry of the screen and theseshould be set so that a square looks square, and a circle round.
Summary:
To see and edit photos with a good degree of accuracy:
Good
VideoCard
16MB of memory, capable of resolutions that match those of themonitor, i.e.: 1280 x 1024 at 75 Hertz with a 24 bit colour.
Monitor 17 inch monitor capable of a maximum resolution of a 1280 x1024 at 75 Hertz or above, and a dot pitch at or below 0.28mm.
Better
VideoCard
32MB of video memory, capable of resolutions that match those of the monitor. I.e.: 1600 x 1200 at 75 Hertz and 32 bit colour.
Monitor 19 inch monitor capable of a resolution of 1600 x 1200 at 75Hertz or above, and a dot pitch at or below 0.26mm.
A final note: most people who spend long hours every day in front of a computermonitor come to realize that a good monitor, and video card, reduce eye strainenormously. Of all the places one can spend money on a computer, spending it on
the quality of the video display should be at the top of the list. While most peoplewill not notice a major difference between the speed of 2 relatively fast CPUs —which can carry a hefty price difference — the differences between a good monitorand video card and bad ones are easily detected.
Many of the newer cameras come with a histogram function. In this article, we takea look at what histograms are, and how they can be used.
Without a doubt, one of the most useful features of digital cameras is the LCD
screen that allows the photographer to inspect a photo immediately after capture.This instant view of the image is one of the reasons digital cameras have been sosuccessful. But, while the LCD screen is near perfect for judging composition andframing, it is not so perfect for judging the exposure.
The reason that current LCD screens are unreliable with regard to exposure isbecause of the way the image is displayed. LCD screens use a relatively small
number of pixels and a bright fluorescent light to reproduce the image. The result isthat it tends to make photos look brighter than they actually are.
Histograms
Histograms are a graphical means to verify the accuracy of the image previewprovided by the camera's LCD screen.
The graph represents the 2 extremes of thebrightness of any photo: the shadows, onthe left, and the highlights on the right.
The distribution of the graph — where thespikes and bulges are clustered — indicatewhether the image is too dark, too bright, orwell-balanced.
Some examples:
In the samples shown below, the brightness or the image is reflected by itshistogram on the right. Note that the graphs actually contain spikes throughout. Butwhat is important is to observe the position where the bulk of the brightness issituated.
With an image depicting a subjectwith evenly distributed brightness, the
image's histogram will have adistribution of brightness that will bemost prominent near the centre partof the graph.
An underexposed image's histogramwill have a distribution of brightnessthat tends to be mostly on the left of the graph.
An overexposed image's histogramwill have the bulge showing thebrightness distributed mostly towardsthe right of the graph.
It is important to understand here that not all images have to exhibit this bulge in
the centre part of their histogram. Much depends on the subject of the image. Insome cases, it might be appropriate for the histogram to show a dominance at oneend or the other, or both.
The histogram of the photo below is an example of this:
Notice that the histogram of the image atleft shows large spikes on the left andright, an indication that the image mightcontain extremely bright and extremelydark areas. The histogram reflects thefact that the image is almost 2-toned,with very bright and very dark areas. Inthis case however, it is normal and theexposure is correct.
Using the information provided by the histogram
Generally, the histogram is a reliable way of deciding whether or not the camera'smetering evaluated the subject accurately. Should the histogram show an over- orunder-exposure, exposure compensation should be used to correct the situation.
Exposure compensation is a system that allows adjustment of the camera'smetering for those situations that can lead it astray. Certain types subjects arenotorious for fooling camera meters into producing an inappropriate exposure.Snow, for instance, often causes meters to under expose the image.
No exposure compensation With +1.5 EV CompensationIn the case of the photos shown above, the effect of the exposure compensation isto brighten the photo and make it more lifelike. By design, the metering of thecamera tries to ensure that the bulge of the luminosity in the image falls around themiddle. However, in this case it isn't appropriate, and the bulge should occur on theextreme right, as befits bright and reflective snow.
While some of the more advanced metering systems can take these situations intoaccount, most metering systems do not, and every metering system willoccasionally misbehave.
Should a histogram indicate an under-exposure, the exposure compensation
should be set towards the positive (+0.5 or + 1EV) and the photo re-taken withthe compensation. Ideally, the process should be incremental so that the result of each change can be observed.
Should the histogram indicate an over-exposure, then the reverse has to beapplied and the exposure compensation should be set towards the negative side(-0.5 or -1EV), causing the camera to use either a smaller aperture, or a fastershutter speed.
It becomes obvious that if the histogram is analysed in the context of the subjectof the image, it can be very useful to adjust the obtain a correct exposure:
For a subject that: The histogram should be:
is composed of an averagebrightness
mostly showing the brightness in themiddle of the graph.
contains dominant dark areas mostly on the left
contains dominant bright areas mostly on the right
and adjustments to the exposure compensation made so as to reposition thedistribution of brightness in the histogram closer to its expected position, and
Besides the nearly unavoidable "which camera should I buy?" question, another one
that regularly crops up in the mail we receive at megapixel.net is : "how longshould I expect a digital camera to last?" .
It turns out to be an easier question to answer than the first question, but one thatcannot truly be answered with any great degree of precision. After all, when onestarts to think about it, the question involves more than one component. The firstbeing "how long will the parts last?" , the other being "for how long will it be useful?"
To try to answer the first part, we made a few phone calls and sent out some e-mails. These helped resolve at least one important aspect about the longevity of adigital camera: the life expectancy of an image sensor. The quick answer is "a long,long time". Indeed, according to information provided by a few manufacturers — we
hasten to add here that the life expectancy of their products is not one of theirfavourite topics as they would rather concentrate on their features and quality —nevertheless, under normal use, the life of a sensor should exceed that of its owner.(it would seem that one can assume that it is safe to "gift" your digital camera inyour will). But, as usual, the devil is in the details and one can probably alsoassume that endless zoom photos of the sun at noon will tend to impact the lifetimeof the CCD; while photos of Aunt Martha, or of the household pets, will allow it tooutlast you.
A digital camera is a mix of analogue anddigital parts. The optical elements shouldlast if treated carefully; the analogue bitssuch as zoom and autofocus motors andthe various controls should also have adecent life span if made of good qualitymaterials. The digital parts — theelectronics — could potentially outlasteverything else. However, one elementthat could possibly limit the lifeexpectancy of a digital camera — barringthe failure of one of the componentsmentioned above — is the recordingmedium.
Today, four major types of memories
coexist in the market: CompactFlash(including Types I & II and Microdrives),SmartMedia, Memory Stick and MMC/SDcards (we are purposefully excludingfloppy disks and CD-ROMs). Experiencetells us that smaller, faster and morecapacious systems are bound to comeover the horizon, and replace them,possibly rendering them obsolete.
Here, we can look back at over the 100 year history of photography to help us put
things in perspective. Before 35mm film pushed all others out of the way, mediumformats dominated. Today, half a century later, medium format film can still befound albeit in more "specialized" shops. While it is extremely unlikely that thememory formats of today will still be readily available in another half century, evenif new formats come along to elbow the current ones aside, we should expect them
While knowing that the CCD in your camera will likely outlive you is nice, if experience with other common objects can be used as a guide, something elsemight turn out to be the camera's downfall. After all, we live in a consumer societyin which products are often "technologically" obsolete by the time the consumer canbuy them. Newer and "better" technologies are always threatening to dethrone well-
established ones, and push them onto that ever growing pile of abandoned "state of the art" technologies (remember the 8-track tape?)
1971 Nikon F: Great camera and lethal weapon.
Indeed, in all likelihood, today's digitalcamera will be outpaced by the modelsavailable 2 years from now. This is nothingnew.
A Nikon F was revolutionary in 1968, now it'san antique: a collector's camera. Many "old" Fstill work like a charm, take superb photos,and are solid enough to repel invaders whenspun from their neck straps. But they are no
longer "state of the art"; and their onceproud owners have abandoned them. Inshort, they aren't new . They don't have allthe bells and whistles; they lack Programmodes; they don't have elaborate meteringsystems.
The same will likely happen to today's digital cameras. Just like yesterday'scameras, they will likely work beautifully for quite a long time if they are maintainedproperly. Digital devices tend to be very reliable; and when they fail — at leastaccording to those who repair them — they tend to fail quite soon into their life if
they are going to fail at all. As for the mechanical and optical parts, with some TLC,there are no major reasons they should not last a very respectable period beforesuccumbing to wear.
These facts aside though, the reality is that the digital camera one buys todaystands a good chance of being unwanted tomorrow. By their very nature, digitalcameras are creatures of the digital age: a kind of peripheral of the computer. Thatvery fact implies that much like computers, digital cameras will become as"obsolete" in our eyes as one of the first IBM PCs, or the first Macintosh generation,is today. Yet, just like these "old-timers" that can still run the programs of their day,it is likely that our digital cameras will still function and still take good photos 5 oreven 10 years from now. Odds are it will be their owners — us — that will be eyeingnewer, faster, and more "able" products, long before these can no longer fulfill their
role.
Put another way, we will most likely determine our digital cameras to be obsolete, orinadequate, long before they actually are. So the best advice on the longevity of adigital camera is probably the same as it is for so many other things. Expect it tolast forever, but be aware that you'll be likely to want another, newer one longbefore it actually needs to be replaced...
Light meters
To obtain a correct exposure, all cameras rely on their light meter. Light metersexist in a variety of configurations, but with digital cameras, they can be classified
into two broad categories: those that measure the light falling onto the sensor, andthose that measure the light arriving at the camera itself, by virtue of being placedclose to the lens. Of these two systems, the former is generally more complex andoften more accurate.
TTL Metering
The majority of digital cameras use what is commonly called "TTL" metering. TTL,an abbreviation of "Through The Lens" , and indicates that the light being measuredis the same as the light the sensor receives. This measurement can be made in avariety of ways, the most common of which are: centre-weighted, segmented —also called matrix metering, or spot. Each method defines how the light ismeasured throughout the image area or frame. For normal exposures though, mostcameras rely on either center-weighted measuring or segmented measuring, both of which involve generating an average measure to suit the entire image frame. Spotmetering is usually reserved for specific applications that require a light measure ata precise point, ignoring the rest of the frame.
Centre-weighted metering
Centre-weighted metering, themost common system, evaluatesthe light arriving at the frame andallocates differing levels of importance to the values gathered,depending on whether the valueoriginated in the centre of theframe or its periphery. Theproportion is generally 80% of theoverall measurement comes fromthe centre of the frame and 20%from the surrounding area. This
type of measurement means thatwhatever is measured at the centreof the frame, be it light or dark, willhave a significant impact on theaverage generated and, therefore,the shutter speed/aperturecombination selected to make theexposure.
In short, pointing the camera at something dark, can sometimes result in an imagewhich will be correctly exposed in that specific area but overexposed in others. Thereverse is also possible. This problem usually occurs when the scene to bephotographed presents great variations in lighting, such as a scene that contains
both bright sunlit areas and shadows. When faced with these conditions,photographers using these types of light meters usually obtain a further averagereading, by first selecting a dark zone to get a measure, then measuring a brightzone, and finally averaging the two readings to set the aperture and shutter speedso all areas are exposed correctly.
Most meters can be "fooled" into over orunderexposing when they are presentedwith scenes such as snow or backlitsubjects. The photo at left is a case in point.Here, the camera's meter was trusted toselect the proper exposure for this snowy
scene. The intense white of the snow causesthe meter to underexpose the picture,resulting in a photo that looks as though itwas taken at dusk. This is a commonmetering fault, and occurs across the boardwith all types of metering systems.
In cases such as this, an exposurecompensation control is a useful asset. Itallows the photographer to "dial in" extralight into the image. The unit used tomeasure exposure compensation is the EV(Exposure Value). Many cameras, digitaland otherwise, offer this feature.
Both photos taken with the Epson PhotoPC 700.
Here, the same picture was taken withthe the camera's exposure compensationset to + 1EV. The resulting image isnoticeably brighter, the snow is white asopposed to grey and the impression of animage taken at dusk is gone.
In the case of the photos presented here,
the problem is the overwhelming white of the snow impacting the meter's reading,and therefore the aperture and shutterspeed the camera selects. The reversecan also be true. Situations where theforeground is brighter than thebackground can be difficult for a lightmeter to evaluate correctly. Often, theresulting photo will be less thansatisfactory, the background being toodark. Using exposure compensation tolessen the importance of the foreground
can improve the image significantly. Forexample, setting the camera to minus –½EV, or –1 EV, can have the effect of brightening up the background, withoutoverexposing the foreground.
The effect of Pre-programmed Modes
Camera programs, as their name implies, are programed choices which willpreference either the shutter speed or the aperture depending on what is wanted. A
"sport" or "action" program will give priority to the camera's shutter speed overthe depth of field, so as to freeze fast movement; a "landscape" or "scenery"program, on the other hand will favour the aperture selection over the shutterspeed, so as to provide a greater depth of field. Both variables are linked: morelight (larger aperture) means a faster shutter speed, more depth of field (smaller
Whatever the program, it should not cause an exposure error. However, using aprogram inappropriately can affect the resulting image. If an aperture priorityprogram is used in a setting that does not offer sufficient light, the low shutterspeed forced by the program can cause camera shake, resulting in a blurry photo.
How ISO Relates to Exposure
The ISO ratings for film speed — the speed at which the chemicals of a film willreact to light, or their sensitivity to light — has been carried over to digital imagesensors. All digital camera manufacturers provide the ISO equivalent for the sensorthey employ in their cameras. The smaller the ISO number, the "slower" thesensor, therefore, a sensor rated at ISO 100, would react more or less the same asa ISO 100 film. This also means that an ISO 40 sensor will respond more slowly tolight than an ISO 100 sensor, requiring either a slower shutter speed or a brighterlens aperture than the ISO 100 sensor. The ISO rating of the sensor in a digitalcamera limits the exposure range of the camera since it acts as the base from whichall exposure decisions are made.
Finally, like films, image sensors can be "pushed". In other terms, their sensitivitycan be electronically enhanced, so their ISO rating increases. It is common to seedigital cameras rated in an ISO range, for example: 40-80 ISO or 60-120 ISO.Similarly to film, "pushing" a digital camera to its highest ISO rating can allow thephotographer to capture images that would have been impossible otherwise, butcan have unwanted effects on the image if used incorrectly. When film is pushedto a higher ISO rating, it often causes the grain of the image to become morepronounced. With image sensors, increasing their sensitivity can cause pixelation,colour shifts, and other unwanted side effects to appear in the image. It is thereforeadvisable to experiment with the camera, to see when, and in what circumstances,the higher ISO rating is best applied.
Many compact digital cameras offer a number of shooting modes. This article isintended to clarify the use and functions of modes that are often referred to as agroup as P/A/S/M modes, shorthand for Program, Aperture Priority, Shutter Priorityand Manual mode. Unlike the ubiquitous Auto mode which usually leaves thedecision making for most photographic settings to algorithms, the P/A/S/M modesgive the user some measure of control over how an image is captured, and eachoffers control over distinct aspects of a camera.
Program Mode (P)
Program mode is very similar to the simplest shooting mode: Auto mode. Indeed if some manufacturers use these designations interchangeably as both the Auto and
Program modes let the camera control aperture and shutter speed, the Programmode is supposed to provide some controls over other photographic parameters.Most Program modes allow the user to choose alternative settings for white balance,exposure compensation, metering pattern, and with some cameras, focus pointselection.
Some cameras also offer a souped up version of the basic Program mode. Oftencalled Program Shift mode, it lets the user choose from a range of alternativecombinations of aperture/shutter speeds that would result in photos with generallysimilar exposure characteristics, but which would give preference to freezingmovement (faster shutter speed and wider aperture), or to depth of field (smalleraperture and slower shutter speed).
For example, an aperture of f8 at 1/125 sec would produce an image with a large area in focus — a long depthof field — but could still freeze a relatively slow moving subject. While an aperture of f2.8 and a shutter speed of
1/750 sec. would allow the background to fall out of the focus zone but freeze most fast movement.(The effect has been exaggerated in the images shown here)
Typical uses:
While the Program mode ensures that the camera will select a combination of aperture and shutter speed that will produce a well-exposed image, it still allows thephotographer to select other important parameters. For instance, the camera's Autowhite balance may not be ideal for the lighting environment, and the user can makethat correction.
With a Program Shift mode, the user gets the opportunity to tweak the Program tosuit the subject without having to switch to either of the Priority modes. The processis fast and usually very efficient, so long as the consequences of any shift areunderstood, and should always result in a well-exposed image.
Aperture Priority
As the name indicates, this mode lets the user select the aperture while the cameramatches it to a shutter speed. Control over the aperture means control over the
depth of field in the image: the zone in the image that is in focus.
The rule is the larger the aperture (small f-number values) the smaller the in-focuszone in the image that will be; while the smaller the aperture (bigger f-numbervalues), the bigger the in-focus zone will be.
Shutter speed 1/400 sec.The fast shutter speed froze the skater, but required an aperture of f2.8 which
produced a short depth of field.
Although somecameras allow for longexposures — 5seconds or more —using their shutterpriority modes, theseare usually not ideal
for night photography.
Since, by definition, ashutter priority modeleaves the selection of the aperture to thecamera and thecamera attempts tocapture a life-likeexposure, it can benearly impossible tocapture an image thatis brighter thannormal, somethingthat is usuallydesirable with nighttime scenery.
Typical uses:
Shutter Priority is well-suited to action or sport photography, and even capturingclear images of active children. However, unless there is plenty of light, a fastshutter will usually require a fairly wide aperture to allow as much light as possible
Manual mode is a full control mode.With it the user gets to choose bothaperture and shutter speed whileguided to a proper exposure by themetering. While all other parameterscan also be specified, exposurecompensation is not available sincethe same effect is achieved with slightvariations in either the aperture or theshutter speed.
Manual mode is particularly usefulwhen an external flash is used, as itcan ensure synchronization, meaningthat the shutter will be open while theflash fires.
Manual mode is ideal for capturingphotos at night or to obtain special
effects such as over or underexposures.
Aperture f2.8, shutter speed 1/2 sec.
Typical uses:
Unlike the shutter priority mode, the Manual mode is ideal for long exposures andnight time photography. Since both aperture and shutter speed are under thecontrol of the user, it is possible to override the metering and produce photos that
are brighter than what the camera would yield otherwise.
Similarly, the Manual mode is ideal for images that are captured under perfectlycontrolled lighting conditions, such as a studio environment.
Conclusion
Understanding the function and the use of P/A/S/M modes offers the user a
