JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

An Autocollimator for Precise Measurement of the Flange FocalDistance of Photographic Lenses*

M. G. TOWNSLEY AND P. C. FOOTEBell & Howell Conmpany, Chicago, Illinois

THE flange focal distance of a lens is definedT as the distance from the mounting surfaceto the back focal plane. This distance is of con-siderable importance in photographic lenseswhere it is necessary to have interchangeabilityof lenses from one piece of apparatus to another,or of a multiplicity of lenses on a single piece ofapparatus. It is usual to measure the flangefocal distance with- a microscope focused firston the flange plane and then on the image of aninfinitely distant object. Either an object at aphysically great distance or a reticle in the focalplane of a collimator may be used. It is alsopossible, although not so commonly done, tomeasure the flange focal distance by means of areticle in the back focal plane and a collimatedtelescope.

It may be pointed out that the flange focaldistance of a lens may not necessarily be identicalwith the flange to film distance of the camerawith which it is intended to be used. In the firstplace, the "back focal plane" may not be aplane, but a curved focal surface. The flangefocal distance, at least in the instrument to bedescribed here, is measured on the axis, and itmay be necessary to introduce a correction totake into account compromise focusing due tocurvature of field. Also, it is usually desirable, inthe case of short focus lenses having fixedmounts, to set or face the lenses for the hyper-focal distance.

Some time ago, it became necessary for one ofthe authors to measure the flange to film dis-tance and the flange focal distance of a group oflenses under conditions such that the back focalplane was inaccessible. Specifically, it was de-sired to determine the position of film runningin a motion picture camera. The instrumentwhich was developed for this purpose has beenrefined and improved to broaden its usefulness,and is the subject of the present paper.

* Presented at the thirty-first annual meeting of theOptical SocieLy of Anierica, October 3-5, 1946, New York,New York.

Because of its history, the instrument hascome to be known in our laboratories as the"Film Focus Collimator." The name "micro-focuser" has been suggested as more appropri-ate, to denote that it is a tool which is primarilyused for accurately focusing lenses. Basically, theinstrument makes it possible to use a standardmicrometer spindle to measure the position of anoptical image by providing a means for accu-rately determining when the spindle face of themicrometer is in the plane of the image.

The optical system is shown schematically inFig. 1. The optical and mechanical arrangementis designed to be flexible and precise. As shownin Fig. 1, the microfocuser has a lamp, a sourcereticle, an axially adjustable objective, and amicrometer. The lens under test is locatedbetween the objective and the micrometer. Be-tween the source reticle and the objective isinterposed a beam-splitter which partially re-flects any light entering the objective into aneyepiece tube at right angles to the main bodytube. A reticle in the eyepiece tube defines thefocal plane of the eyepiece, and is placed withrespect to the beam splitter so that it is imagedin the plane of the source reticle. Thus, anyimaging ray entering the objective in such amanner as to form an image in the plane ofthe source reticle will also be imaged in the planeof the eyepiece reticle. The location of the eye-piece reticle is described in this way rather thanby simply saying that it is in the principal focusof the objective because, for some applicationsof the instrument, the objective is displacedaxially from its infinity focus position.

Figure 2 shows the form in which the instru-ment has been used in our laboratories, assembly,and inspection departments. The lamp is a1000-watt motion picture projection lamp, placedin the lamphouse at the extreme left of the figureand cooled by a suitable fan. The source reticleis placed at the left end of the body tube andconsists of crossed pairs of closely spaced linesruled in opaque silver. A cubical glass block

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VOLUME 37, NUMBER 1 JANUARY, 1947

AUTOCOLLIMATOR

LL' IS

d

FIG. 1.

with a liagonal interface coated with a semi-reflecting coating forms the beam-splitter. Theobjective is a doublet of 18 inches focal lengthhaving a clear diameter of 2- inches. The eye-'piece tube is folded for convenience, and is

equipped with a dark line eyepiece reticle and a

one-inch eyepiece. At the right of the body tubeis seen the mount for the auxiliary lenses which

will be described later, and the mount for thelens under test and the micrometer spindle.The entire unit is mounted on an optical benchconsisting of a pair of heavy cylindrical bars

supported on a pipe and cast iron framework,this form of optical bench being standard inour organization.

The diameter of the objective is large enoughto clear all lenses normally supplied, and suffi-

cient illumination is provided for the smallestaperture to be used. The focal length of thecollimator objective is longer than any lens nor-mally tested, to obtain adequate magnification.

The mount for the, lens to be tested has appro-priate means for attaching the lens to be tested,

and for locating the lens from the flange orshoulder from which the flange focal distance is

to be measured. A micrometer spindle is attachedto this mount, coaxial with the lens to be testedand adjusted so that the distances indicated onthe micrometer scale are measured from the seatagainst which the mounting flange rests. Theface of the spindle is polished accurately squarewith the spindle axis, and flat within one or twowave-lengths.

To measure the flange focal distance, the lensto be measured is placed on the lens mount,

facing the objective L (Fig. 1). The objective L is

set to the position which produces an image ofthe source reticle R8 at infinity, so that the lightbetween the objective and the lens being meas-ured is collimated. The lens L' under test, willthen produce an image -of the source reticle in

its back focal plane, at I'. The face of themicrometer spindle acting as a mirror, and beingat a distance d from the focal plane I', will form

an image Im' at a distance 2d from the focalplane. This reflected image now becomes anobject at a distance u" =f'+2d for the lens L'being measured, which produces an image I" ata distance v" in accordance with the classicalequation for object and image. This image I" inturn, is an object for the objective L which formsan image R3' at a distance v"'. It is obvious that,if the micrometer spindle is moved to reduce d,u" approachesf, v" approaches infinity, and v"'approaches f, bringing the resultant image RsI ofthe source reticle closer to the eyepiece reticle Re.It is also clear that 2d approaches zero twice asfast as the micrometer spindle face approachesthe focal plane of L'. It is this doubling of thefocusing effect which gives the instrument itsextreme sensitivity.- When the source reticleimage comes into focus in the eyepiece reticleplane, the flange focal distance may be readdirectly from the micrometer spindle. Parallaxelimination between the source reticle image R3 '

and the eyepiece reticle Re is helpful, but notessential, for accurate focusing.

The relationship between the movement ofthe micrometer spindle and the movement of theeyepiece image may be more elegantly developed

11

43

M. G. TOWNSLEY AND P. C. FOOTE

FIG. 2.

by considering the lenses L and L' as com-ponents of a single lens having a focal' length F.Then

1/U"+ 1/V"' = 1/F,-u-2 du"-v-2 dv"' = 0,

dv"'Idu" = V///2 /U /2;

we may call dv"'/du" the longitudinal magnifica-tion but, M= v"'/u" where M is the lateralmagnification whence, dv"'/du" = M2. As we havealready seen, if d is the movement of the microm-eter spindle,

du" = 2dd,and therefore,

dv"'/dd = 2112,

so that the microfocuser has twice the longi-tudinal magnification of a telescope or micro-scope of equal lateral magnification.

The instrument as described and as shown inFig. 2 has been used to measure the flange focal

distance of a considerable number of motionpicture camera lenses for the purpose of facingthe flanges to a fixed standard flange focaldistance and for inspecting the lenses afterfacing.

In addition to facing lenses to fixed flangefocal distances, the instrument is equipped forthe calibration of focusing scales. Two methodsare used to effect this calibration: either theobjective is racked in to produce an image atthe desired distance, or the objective is set atinfinity and an auxiliary lens is used.

In the first method, the objective is racked intoward the source reticle by a predeterminedamount, to produce a virtual image at a distancev to the left of the objective, which then becomesan object for the lens under test at a distanceD=v-u+A, where A is the distance betweenthe reference plane of the lens being tested andthe source reticle. In motion picture practice,

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AUTOCOLLIMATOR

D is always measured from the film plane, sothat A is the distance from the source reticle tothe nominal position of the micrometer spindle.The micrometer spindle is set to the flange focaldistance with the lens focused in its infinityposition, and the lens is moved by means ofits focusing mount until the source reticle issharply imaged in the eyepiece reticle plane. Thelens under test is now focused for the distance Dand the point may be suitably marked on thefocusing scale.

To facilitate accurate repetition of the settingsof the objective in calibrating focusing scales,it has attached to the lens cell a precise dialindicator reading in .001". The nose of theindicator bears, through a hinged block, on theend of a rod which is secured to the main bodytube, parallel to the axis of the tube. A series ofthese rods is mounted in a pair of rings whichrotate around the tube so that any one of therings may be brought opposite the indicator.The lengths of the rods are adjusted to give adesired series of positions of the objective andcorresponding image distances D. If the indi-cator is adjusted to read zero when in contactwith the infinity rod with the objective accu-rately focused for infinity, any desired distancemay be set off by rotating the nest of rods tobring the desired rod opposite the indicator andadjusting the objective tube to again bring theindicator to zero. All of the commonly engraveddistances from infinity to 15 feet are providedfor in this way.

For distances closer than 15 feet, the move-ment of the objective becomes prohibitivelylarge, and it is necessary to resort to the use ofauxiliary lenses. The objective is set to theinfinity position and auxiliary lenses are intro-duced between it and the lens being calibrated.A negative element causes 'a virtual image ofthe source reticle to be presented to the lensunder test as though there were a real object atthe required distance. A separate auxiliary lensis required for each distance D. It would bepossible to use fewer auxiliaries and readjust theposition of the objective for some distances, butthis does not lend itself well to production use.

Figure 3 shows the optical layout of the systemwith an auxiliary lens in place for calibratingthe focusing scale at a distance D. The distance

t ~~~~OBJECTIVE AUXILIARY LENS BEING IMAGELENS LENS CALIBRATED PLANE

FIG. 3.

is measured from the image plane, in the sameway as when the objective is racked in. It isnecessary, therefore, that the position of theimage plane be fixed and that the auxiliaries belocated to produce their virtual images at theproper distance from the image plane. A distancefrom the auxiliaries to the image plane has beenarbitrarily chosen, large enough to give adequateclearance for the longest lens to be inserted forcalibration. For convenience, the auxiliaries aremounted on a large wheel, carrying enoughstations to supply one for each distance to becalibrated from 15 feet down to one foot. Theauxiliary lenses are more than one foot from theimage plane, so a positive lens is used for thisshortest distance, forming a real image of thesource reticle in the correct position.

Accurate readings reqiire considerable ac-curacy in the construction of the componentparts of the instrument. To avoid astigmatism,it is necessary that the objective and the beam-splitting prism block be accurately made andfree from strain or warpage due to cementingor mounting. The unused faces of the block areground and blackened to eliminate stray re-flections. For maximum sensitivity, the collimatorreticle consists of crossed pairs of lines ruled inopaque silver and separated by a distance whichwill make them subtend approximately 3 minutesin the eyepiece. The eyepiece reticle is preferablya dark 60° cross.

Original collimation is accomplished with theaid of an accurately flat autocollimating mirrorand an auxiliary telescope, by first using thetelescope to bring the eyepiece and source reticlesinto common focus, and then using the mirrorto establish the infinity setting of the objective.The accuracy of the auxiliary lenses and theirsetting is critical. Each lens is mounted in anadjustable sleeve so that it may be shifted tomake the final focusing adjustment, but mostof the adjustment was made by polishing the

45

AND P. C. FOOTE

lenses stronger or weaker to bring them withinthe relatively short focusing range of thesemounts. The difficulty of making this correctionincreases directly with the focal length.

The adjusting of the power of the auxiliarylenses and their final setting in the mounts wasaccomplished by using a long focus, high aper-ture, lens in the position of the lens to be tested,and a high power microscope on the image.A suitable target was placed at the requireddistance, and the microscope adjusted to focuson its image. The microfocuser and the auxiliarywere then switched in, and the auxiliary ad-justed to bring the image of the source reticleinto the proper plane.

The accuracy with which settings of theinstrument can be made depends on the apertureand focal length of the lens being tested. Withinthe range ordinarily used for motion pictures,it is usual to repeat results between observerswithin -4.0005 inch per inch of equivalent focallength. If large aberrations exist in the lensbeing tested, it is obviously impossible to makeaccurate readings because there is no sharpimage on which to focus. The magnification ofthe system is sufficient to show up poorly cor-rected or defective lenses.

It was said in the introduction that thepredecessor of the present instrument was origi-nally developed during an investigation into thelocation of film in a running motion picturecamera. The present instrument is still usedoccasionally for the same purpose. It is relatively

easy to make accurate settings using the surfaceof the film itself as a "reflecting surface," insteadof the micrometer spindle. In this case, the film

-does not form an actual reflected image, butbecomes a receiver of an out of focus image andan out of focus diffuse radiator of this image,so that the doubling of the defocusing effect ispresent in the same way. Measurement of thefilni position is made by a transfer method.The instrument is used to observe the imageon the film with the camera lens set to infinity,and the focusing is done by adjustment of theobjective of the microfocuser. The camera lensis then transferred to the micrometer adapter,and, without disturbing the setting of the micro-focuser, the micrometer is advanced to bringthe image into focus in the eyepiece reticle plane.The micrometer setting is then the distance fromthe mounting flange to the film plane.

The instrument which has been described herehas proved in practical use to be more accurateand easier to use than conventional microscopemethods. It has been found possible to trainunskilled operators to use the instrument withina few minutes to the point where' they obtainedbetter repeatability of settings than could beobtained by the most skilled operator using aconventional microscope. In one test, a group offour unskilled operators showed less scatteringof settings among the group than a single skilledoperator using a microscope showed among hisown settings.

46 M. G. TOWNSLEY