The Status and Problems of Air Photography

Abstract

The lecture deals with the subject of image qualIty in air photography, exciuding mapping techniques as such. Though pOInting out that ‘air photography’ as a technique includes the aeroplane ond ,ts navigation, II concentrates on aspects of general photographic interest. The fundomentol needs and problems of oir photogrophy were seen by L. P, Clerc in a Trail- Taylor Memorial Lecture delivered ,n 1921. Progress since that time hos been brought about by taking advantage of technological advance in sensitive materials, optics electronics etc. rather than by any fundamenlIy new ideas. Panchromatic emulSIons are (aster, more hIghly colour-sensitized and keep beller. Improvements in their physical hardness have ollowed the development of rapid processing techniques. A machine developed by the Royal Aircraft Establishment processes standard types of 70 mm. (11m (e.g., HP. 3.) at 80 (I. per minute for a power consumption of Six kilowatts. Size is 5 ft, 6 in. long by 5 ft. 810. high. Solutions at 110 Fare applied by high pressure ets; dryto- day time is two minutes with washing to commercial standalds. Emulsions having the property 0 constant speed whIle controst varies with development time would be more useful than current emulsions in which the speed varies while contrast remains substantially unchanged. The luminous density relation for effective photographic light versus solar attitude can give serious over-exposure in air photography of desert areas, and it has been shown by photography and photometry that exposure settings for such areas should be based on the illumination on the horizontal plane, which was abandoned for ground photography several years ago. Lenses, particularly wide-angle lenses for survey, have been greatly Improved, both in definlt,on and 10 illumination off-axis: thus it Is possible and pro(ltable to use slower flims with consequently improved angular resolution. Examples are shown of the effect of lighting angles, which though out of control are very Important ;n air photography. The alfcraft camera is briefly compared with lhe eye and the television camera, It Is less sensitive than eIther, but records more IOformation and has the advantage of buit-,n slorage. The latest wide-angie survey cameras cover 120, comparable with the cover of the eyes, at slightly lower angular resolution. Brief reference is made to the coming substitution of resolution tests by “frequency response” or “contrasl-lransfer” methods. Experimental evidence is given of the value of such methods ,n predicting accurately the effect of movement on image quality.