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Aviation History
1947
1947 - 2194.PDF
.,-7" FLIGHT The L 8 air camera of 1917. The magazini tes. T-IE Central Photographic Establishment at Bensonheld a P.R. Conference from December ioth toDecember 19th, which was attended by representa- tives of all three Services of the British Empire, and of our Allies in the late war. The ground covered was very extensive, and a feature of the conference was an exhibition of cameras, equipment and technique, held in a hangar. C.P.E. is responsible for all strategical photography re- quired by the three Services and, in addition, now does a large amount of air survey work for the civil Ministries. The flying side is manned entirely by the Royal Air Force, but both the Army and the Navy are represented in the ground work. All the flying squadrons are stationed at Benson with the exception of No. 82, which is at present busily engaged in East Africa with a big programme of Colonial survey- Reconnaissanci Modern Aids to P.R. on Show All phases of wartime and peacetime air photography were on show at the exhibition, including a display of non- standard cameras used for special tasks. Most of these cameras are basically standard in that the film is moved for each exposure and "take" is by releasing a focal-plane shutter. In some cases mirrors are used to get oblique photographs, fore, aft or sideways; lenses of various focal lengths from 3J in to 50 in can be fitted to get the correct scale according to the height at which the aircraft has to fly ; but in their elements these cameras aTe standard pro- ducts. The problem of overlap photography at low heights by modern high-speed P.R. aircraft has, however, presented a problem of considerable magnitude. The forward speed of the aircraft is now so great (500 m.p.h. = 733 ft per sec) that there is no time to wind-on the film, reset the shutter and make the exposure so that each photograph overlaps the preceding one by the 60 per cent necessary for stereo- scopic purposes. Moving-Film Technique At medium heights this trouble is overcome by carrying '' Master'' and '' Slave '' cameras. These take alternate photographs the '' Slave '' camera filling in the gaps which would be left by the '' Master.'' For high-speed reconnaissance at low heights such as, say, 300 ft, the most ingenious means have been devised. No shutter is fitted to the camera. Its place is taken by a narrow transverse slit in the focal plane. The film is made to pass continously over this slit at the same speed as that of the image of the ground beneath, as projected by the lens. During the passage of the image over the slit, while the film is being exposed, the change of angle is so small that no visible distortion occurs if the' aircraft maintains steady flight at the predetermined speed. This standard of perfection is, of course, impossible, but the results obtained by a good pilot under non-bumpy con- ditions are remarkable for their clarity of detail.
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