MARCH, 1942

Lenses for Aerial Photography*

R. KINGSLAKEEastman Kodak Company, Rochester, New York

THE earliest photographs' from the air wereT made in France, where aeronautics hasalways exerted a fascination for the man ofingenuity and enterprise. As far back as 1856,M. Nadar took photographs over Paris from acaptive balloon, using (presumably) wet collodionplates. Later, in 1885, Tissandier and Ducomused dry plates from a free balloon. Otherpictures, somewhat unsuccessfully, were madewith cameras supported by a kite. Our earliestrecord of the use of aerial photography inmilitary work was the photography of enemyfortifications in China from a balloon by ColonelRenard, in 1900. Apparently, the first photo-graphs to be made from an airplane were takenby Wilbur Wright in April, 1909.2

PHOTOGRAPHY IN THE WORLD WAR, 1914-18

Just prior to the commencement of the war of1914-18, one or two workers in the French army,notably a Captain Saconney, were attemptingphotography from airplanes, but very littleofficial interest was shown in the matter. How-ever, after a Zeppelin captured on August 22,1914 was found to be equipped with a camera,official interest was suddenly aroused, and theFrench army at once established a regular aerialphotographic service, their example being fol-lowed soon after by the British and other Allies.By the end of the war, photography had become amajor war activity, and tens of thousands ofphotographs were made every week. It is esti-mated that by 1918 over a million prints amonth were being made and distributed.

At first, any convenient camera was held in thehand, and merely pointed over the side of theplane at the object to be photographed. However,in 1914, when the demand for aerial photographsincreased by leaps and bounds for war purposes,it became apparent that special cameras were

*Presented at Symposium of Optics in the NationalDefense at the twenty-sixth annual meeting of the OpticalSociety of America held in New York City, October 24-25,1941.

1 L. P. Clerc, Applications de la photographic aerienne,Chap. I.

2 D. M. Reeves, Aerial Photography, p. 3.

necessary. An aerial camera must be very rigid,mounted so as to be free from vibration, and pre-focused for infinity but not thereafter adjustablefor focus. It must be equipped with a long-focused lens to show fine detail, and at first anadjustable focal plane shutter was used as nolarge between-lens shutters were available. Also,the size of picture should be as large as con-venient to cover a wide angular field.

Prior to about 1922, aerial photographs wereinvariably made on glass plates, housed in vari-ous ingenious forms of magazine or changing boxholding from 6 to 50 plates at a time. At first,roll film was found to be unsatisfactory, and itwas not adopted for aerial cameras until two orthree years after the war had ended. Theproblem of the use of roll film was twofold:firstly, adequate processing equipment had to bedesigned to handle large rolls of film, andsecondly, some means had to be found to hold thefilm flat during exposure. A pressure pad andglass plate have been used, but this is likely tocause scratches and static marks on the film; amuch better solution was found in Folmer'sintroduction of a "suction back," to pull the filmdown against a flat perforated metal plate. 3

The French4 adopted from the first a focallength of 25 cm- (10 inches) and a 13X18-cmplate (5X7 inch). The 10-inch focus was soonfound to be too short for high flying, so cameraswere constructed for 50-cm (20 inch) focus,covering an 18 X 24-cm plate (7 X 9-2 inch), whichbecame the standard focus and plate size for theFrench army. The speed of all these lenses wasf/4.5. A little later, the French introduced a lensof 120-cm focal length (47.2 inches), also for the18X24-cm plate, at an aperture of f/6.5, claimingthat this was the longest camera in the world!This camera was not particularly popular as itoccupied far too much space in the plane, andcovered only a very narrow angular field.Towards the end of the war, a moderately wide-angle camera was introduced covering an 18 X 24-

3 U. S. Patent 1,309,798 (1919).4 A. H. Carlier, La photographic aerienne (1921) Chap. 2.

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J. . S. A. VOLUME 32

R. KINGSLAKE

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!IK/FIG. 1. Ross "Xpres." Stuart

and Hasselkus, British 29,637(1913).

FIG. 2. The four-piece air-spaced lens (Aviar, Dogmar,Aerostigmat).

FIG. 3. The Zeiss "Tessar" lens(Rudolph, U. S. 721,240).

cm field with a 25-cm lens. This was intended forrapid reconnaissance or survey work.

The British5 Royal Flying Corps began experi-ments in aerial photography as early as 1912, butprogress was very slow until the war started.At that time, 4 X 5-inch glass plates were regardedas standard, and 8-inch or 10-inch focus lenseswere used. The camera was hand-held at first, butlater fastened to the outside of the fuselage of theplane. The operation of the shutter and platechanging mechanism was later made automatic,power being derived from a small air-screwmounted in front of the camera.

Eventually, cameras were designed to takeinterchangeable lenses of 4-, 6-, 8-, 10k-, and20-inch focus, and the 4X5-inch negatives wereenlarged before use. When the war began, noBritish lenses were available in sufficient quan-tity, and Zeiss Tessar lenses were requisitionedfrom their owners by the Government. LaterRoss and Taylor, Taylor, and Hobson com-menced manufacture of the Xpres (Fig. 1) andAviar (Fig. 2) lenses, respectively (bothf/4.5).

Towards the end of the war, all the Alliesagreed to standardize on the 18X24-cm size(7X92 inch) and lenses from 7-inch to 36-inchfocus were used.

The German army during the last war startedwith the very great advantage of possessing afully developed optical glass and lens industry.Their first aerial camera used 9 X 12-cm plates(31X42 inch), with anf/4.5 Tessar (Fig. 3) lensof 25-cm focus. This picture size was soon foundto be too small, and it was replaced by a 13 X 18-cm size (5 X 7 inch). For this size, the standardlenses were of 25- and 50-cm focus (10 and 20

5Photography as a Scientific Implement (Blackie, 1923),Chap. 10.

inch), with the usual glass plates and focal planeshutters. For the 25-cm size, a Zeiss Tessarf/4.5lens was used and for the 50-cm size, an f/4.8Triplet (Fig. 4). Later 70-cm (272") and 120-cm(48") f/7 Triplets were adopted also. TheGerman cameras were made by Zeiss, Goerz, Ica,and Ernemann.

The earliest use of roll film in an aerial camerawas apparently the Douhet-Zollinger (Italian)camera made in 1915, and shortly after aban-doned. It used an f3.5 Zeiss Tessar of 10-cmfocus, giving a 6X6-cm picture. The film 6 cmwide was supplied in 20-meter rolls for 300photographs, a perforation every 6 cm along oneedge of the film indicating the position of suc-cessive frames. The mechanism advanced the filmand simultaneously cocked the shutter, just as insome modern types of amateur cameras.

THE USE OF LONG-FOCUS LENSES

Soon after aerial photography became a routinematter, a very interesting fact gradually emergedand became firmly established. 6 This was, that alens of long focus using contact printing, givesbetter definition of fine detail in the print than asimilar short-focus lens using enlargement inprinting. From simple considerations, we shouldnot expect this to be the case. The shorter-focuslens should give elementary images which aresmaller than those given by the longer-focus lens,in proportion to the two focal lengths; however,if we then enlarge the image given by theshorter-focus lens in inverse proportion to itsfocal length, we should end up with the samesize of elementary images in both.

This discrepancy is apparently due to twofactors. Firstly, the enlarging lens has inevitably

6 H. E. Ives, Airplante Photography (1920), p. 59.

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LENSES FOR AERIAL PHOTOGRAPHY

some aberrations which tend to spoil the sharp-ness of the image, and secondly, the graininess ofthe negative is magnified by the enlarging process,and it is therefore more serious with shorter-focuslenses.

An imaginary example will show how thisoperates. Suppose that for a low-contrast objectsuch as would be ordinarily photographed fromthe air, the resolution of the emulsion is about 25lines per mm. The resolving power of a certain6-inch lens may be about 40 lines, and that of thecorresponding 12-inch lens would, therefore, beonly 20 lines. The combination of 6-inch lens andfilm might resolve about 16 lines, and thecombination of 12-inch lens and film would re-solve about 12 lines per mm. On enlarging the6-inch image by a factor of two to give the samesized print as was obtained with the 12-inch lens,the resolution drops from 16 to only 8 lines,which is much less than that found in theunenlarged 12-inch picture.

Whatever may be its cause, this effect is wellenough established to deter aerial photographersvery definitely from using smaller lenses. A short-focus lens would lead to great gains in filmeconomy, lightness and compactness of equip-ment, speed and efficiency of shutters, and ease infilm processing; all of these advantages are nowbeing sacrificed to obtain improved definition.

This phenomenon is familiar to users of minia-ture cameras, who often find that a 5-timesenlargement from a 1 X 1-inch miniature nega-tive, taken with a 2-inch lens, is never as crispand clean-cut as a direct contact print from a5 X 7 negative made with a 10-inch lens.

DEVELOPMENTS IN THE UNITED STATES

In this country, the Brock7 Aerial Surveyingsystem was started in 1914, using glass plates61 X82 inches in size, with a 9-inch Goerz lens.The pictures were enlarged to the equivalent of a14-inch lens efore making the maps. When theUnited States entered the war, European lenseswere not available, and it became a very graveand urgent problem to initiate and develop to anadequate capacity both the optical glass and theaerial lens industries. The optical glass story isnow a matter of history, but the absence of

7 J. Opt. Soc. Am. 22, 111 (1932).

suitable glass types greatly hampered the designand manufacture of aerial lenses to an extentscarcely realized today. As Mr. Frederick re-marked in his paper read before this Society in1919,8 "We were confronted with the problem ofmaking a lens out of very unsuitable materials, aproblem not unlike that of making something outof nothing, and altogether discouraging to con-template." Finally, when a supply of bariumcrown glass was promised, the design of an f/4.5lens of 8-inch focus (later raised to 10 inches)was completed, to cover a 4 X 5-picture, similar tothe lenses used regularly by the British.

While the f/4.5 lens was being designed, a 20-inch f/6 lens was required, and both designs werecompleted almost together. These lenses were theonly ones to go into regular production byEastman Kodak Company before the war endedin November, 1918. A 48-inch f/7.7 lens wasplanned, and would have been manufactured hadthe war continued. These three Eastman lenseswere all of the 4-piece airspaced type (Fig. 2)made from dense barium crown and barium flintglasses of approximately the same index.

In designing a special lens for aviation pur-poses, advantage may be taken of the unusuallynarrow angular field generally employed. Caremust be taken too to overcorrect the lenschromatically to a slight extent, since almost allaerial photography is performed with panchro-matic film and a blue-absorbing filter.

DEVELOPMENTS SINCE 1918

As soon as the war was over in 1918, the greatrush to produce aerial lenses subsided, anddesigners had time to study the situation andendeavor to stabilize it. From that time threequite distinct trends have appeared, namely:(a) lenses for military reconnaissance and pictorialpurposes, (b) lenses of high aperture for night

1 LV/FIG. 4. The Triplet type of construction.

8 J. Opt. Soc. Am. 2, 34 (1919).

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R. KINGSLAKE

FIG. 5a. The Goerz "Dagor"lens, German Patent 74,437(1892).

FIG. b. Zeiss "Orthometar,"Mert6, U. S. 1,792,917 (1931);Ross "Wide-angle Xpres." Has-selkus and Richmond U. S.1,777,262, (1930).

FIG. 6. Eastman Kodak 13k-inch f/3.5 aviation lens. Altman,U. S. 1,880,394 (1932).

photography, and (c) wide-angle lenses for aerialsurveying. Some signs of these distinctions wereappearing even during the war, but after it wasover, the separation rapidly became very marked.

(a) Military Reconnaissance Lenses

From the early Eastman and Folmer cameras,great strides have been made in the UnitedStates, notably by the Fairchild Corporation, inboth the hand-held 4 X 5- and 5 X 7-types, and theautomatic roll-film 7 X 9- and 9 X 18-inch cameraswith suction back. The 7 X 9-size remainedstandard until about 1935 when it becamegradually replaced by the 9 X 9-inch size. The9 X 18-inch size has not been particularly popular,for some reason. The Fairchild cameras at firstused focal plane shutters, but later they de-veloped a very large between-lens shutter whichhas been much used.

For these cameras during the last 20 years, thestandard focal lengths and apertures have beensomewhat as in Table I. A few other odd sizeshave been used occasionally, such as 5X5 inchand 5 X 6. Practically all the lenses used havebeen of the Tessar type, comprising a pair ofairspaced elements ahead of the stop, and acemented pair behind. The principal manu-facturers have been Zeiss (Tessar, Fig. 3), Goerz(Dagor (Fig. 5a), Dogmar (Fig. 2), and Aerotar),Schneider (Xenar, Fig. 3), Eastman Aerostigmat(Figs. 3 and 6), Bausch and Lomb (Tessar,Fig. 3), Dallmeyer (40-inch Telephoto, Fig. 7),Taylor-Hobson (Cooke, Fig. 4), and Ross(Xpres, Fig. 1). During the present war, thou-sands of aerial lenses of many sizes and types arealready being manufactured in thc U. S. A. fromdomestic glass, a situation in strong contrast tothat obtaining during the war of 1914-18.

Just as in 1914-18, the present war is causing avery rapid development in the art of takingmilitary photographs from the air. With anti-aircraft fire reaching almost to 40,000 feet, areconnaissance plane must either fly above thatheight, or the flier must hide in the clouds untilover his objective, and then dive down to snatcha picture before being hit.

The first of these alternatives requires a long-focused lens such as the 40-inch f/8 telephotonow being manufactured in this country, togetherwith a red or infra-red filter to penetrate atmos-pheric haze. It is interesting to note that in 1914,the usual height for aerial photography was 3000or 4000 feet, and that by the end of the war

I FIG. 7. A typical Telephoto lens (Booth, U. S.1,480,929).

TABLE I.

CorrespondingSize Focus Angular

(Inch) Speed (Inch) Semi-Field

4X5 f/4.5 68 2605X7 f/4.5 9 or 10 240

f/5.6 20 120 Regular andTelephoto

f/8 40 60 Telephoto7X9 f/4.5 10 300

f/5 12 260f/4.5 20 160

9X9 f/4.5 12 280(or 8X10) f/3.5 13iL 250

"1 f/6.3 14 240"1 f/6 24 15°

9X18 f/6 24 230

l i 5

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LENSES FOR AERIAL PHOTOGRAPHY

(a) (b) (c)

FIG. 8 a. Busch's "Pantoskop" lens, f/30, 450 (1865). b. Von Hbegh's "Hypergon" lens,f/30, 670 D.R.P. 126,500 (1900).c. Richter's "Topogon" lens, f/6.3, 450 D.R.P. 636,167 (1936).

reconnaissance planes were forced up as high as18,000 feet.

For the second alternative, a number of small,fully automatic cameras have been developed inEurope, one example being a focal plane camerautilizing a 5X5-inch film between a glass plateand a pressure pad, with an assortment ofinterchangeable lenses. This camera is intendedto be carried in the tail of a small plane andoperated entirely by remote control.

(b) Night-Photo Lenses

Towards the close of the World War, someexperiments were made in night photographyfrom the air, using a flash-bomb dropped fromthe plane. The method was to use a flash of veryshort duration (1/100 second) and hold thecamera shutter open until the picture was taken.In Dr. Ives' book,9 written in 1920, he predictedthat longer flashes would probably be used, withsome arrangement to synchronize the opening ofthe shutter with the peak of the flash. Eventrailing the flash bomb on the end of a longwire had been considered, but dismissed asimpractical!

Within the last few years Colonel G. W.Goddard of the U. S. Army Air Corps hasdeveloped a satisfactory photo-cell synchroniza-tion device by which excellent night flash photoshave been obtained using an Eastman 132-inchf/3.5 lens (Fig. 6). Progress in night photographyis most necessary and will assuredly come both inthe direction of brighter flash-bombs, and by theuse of faster films and lenses.

9 Reference 6, p. 387.

(c) Lenses For Aerial Surveying

In the early days of aerial photography, thesame lenses were used for reconnaissance workand for map-making. It was, therefore, necessaryto correct the distortion particularly well. Sincethe war, however, the needs of the map-makerhave received a great amount of attention, and anumber of special distortionless wide-angle lenseshave been designed and manufactured for thispurpose.

One of the first of these was the Goerz Aerotar,a modification of the well-known Dagor (Fig. 5a),working atf/6.8, covering a 5' X 6-inch picture at6 inches focal length. This corresponds to anextreme half-angle of view of 340.

Within a few years, a focal length of 8- inchesbecame standardized for a 7 X 9-inch picture,corresponding to a corner angle of 341O* At firstlenses such as the f/6.8 Dagor were used forthis, but around 1930 appeared the Zeiss"Orthometar," and the very similar Ross "Wide-angle Xpres" (Fig. 5b), both working at f/4.These two lenses are really a modification of theDagor, obtained by separating the inner positivemeniscus elements from the cemented triplets,and "bending" them to a somewhat weakershape. The outer doublet members have a veryweak negative power, so the whole system can, ina sense, be regarded as a modified periscopic lens,the inner lenses constituting the periscopic part,the outer components being practically afocalcorrecting lenses to remove the aberration resid-uals of the positive elements. The field of this lensis satisfactory to almost 35° at f/4, and quitedistortionless.

l Al Al ll...t

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WALTER G. DRISCOLL

These lenses represented the ultimate in aerialsurvey lenses until the Zeiss "Topogon" f/6.3lens (Fig. 8c), covering a half angle of 450 wasdesigned by Richter in 1933. A 6-inch lens of thistype covers successfully to the corners of a 9 X 9-inch picture. It is also made in this country byBausch and Lomb, as the "Metrogon" lens. Asample in the 12-inch size has been made to coveran 18-inch square picture, but this very largesize seems likely not to be used extensively be-cause of the size and weight of the camera, andthe difficulties of handling this large film. Ross inEngland have made a slight modification of theTopogon lens, known as the f/5.6 Wide-angleSurvey lens, by dividing the rear positive elementinto two airspaced elements.

As can be seen from the diagram in Fig. 8, theTopogon lens is of an extreme meniscus type ofconstruction, being probably descended fromthe old Harrison "Globe lens," or the Busch"Pantoskop," making use of the teaching givenus by Von HJegh in the Goerz Hypergon. The

MARCI, 1942

deep meniscus form of the Hypergon leads to aperfectly flat field out to 670 from the lens axis,but as it is uncorrected for chromatic andspherical aberration, its useful aperture is limitedto about f30 or less. The Pantoskop wasachromatic but not spherically corrected, itsaperture being likewise limited to aboutf/30. InRichter's design he has separated the positiveand negative elements of the Pantoskop lens, andgiven the negative elements a "bending" to adeeper meniscus shape. In this way, sufficientdegrees of freedom are made available to correctthe aberrations atf/6, over a semi-field of about450. At this very wide angle, the illumination isnaturally rather low, but the lens is quitesatisfactory in practice.

A high lens speed is of almost no importance tothe peace-time mapping photographer, becausehe flies only in perfect weather. However, extremeangular fields are very desirable, and now that a450 field is available, he will never be satisfiedwith less.

J. 0. S. A. VOLUME 32

Optics in Sabotage and Espionage*

WALTER G. DRISCOLLFederal Bureau of Investigation, Washington, D. C.

THE maintenance of our national defense andT the preservation of our internal securityhave increased the quantity of work that is beingperformed by the scientific crime detection labo-ratory of the Federal Bureau of Investigation inWashington, D. C. To meet the increase andinflux of investigations arising from sabotage andespionage activities, the laboratory has availablenumerous pieces of equipment and precisioninstruments that will aid in the advancement ofthis purpose. The laboratory of the FederalBureau of Investigation, being a service organiza-tion, offering assistance willingly in all mattersinvolving scientific crime detection, presentsitself to the cause of doing all in its power to

* Presented at Symposium of Optics in the National De-fenise at the twenity-sixth annual meeting of the OpticalSociety of America held in New York City, October 24-25,1941.

analyze and solve, limit and prevent any criminalactivities which are injurious to this country'sinternal security.

The laboratory staff is made up of many men,each one a specialist in a particular branch ofscience. The forensic applications of petrography,metallurgy, chemistry, serology, toxicology, spec-trography, physics, firearms identification, anddocument identification are numerous, and theyform the basis of the work of the laboratoryspecialists in the respective fields. Needless tosay, optical equipment is not found lacking.Optical instruments are probably used morefrequently than any other type of instrument.Optical instruments, because of their ability toreveal characteristics minute in size and manytimes significant in nature from the standpointof criminal work, and to expose other charac-

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