In the ordinary photography of stationary objects, exposure is a variable entirely at the operator's command. Plates of any speed may be selected, so that attention may be focussed on latitude, color sensitiveness, and other tone rendering characteristics. The exposure may be made of a length sufficient to insure all the useful photographic action lying in the "correct exposure" portion of the sensitometric curve. The exposure ratio of any filter it is desired to use is a matter of indifference—its effect on color rendering need alone be considered.
Airplane photography is sharply distinguished from ground "still" photography by its severe limitations as to the amount of /the exposure. The actual duration is definitely restricted by the high speed of the plane. In peace work this can be offset in part by using slower planes or by flying against the wind. The practical limitation to -nrrr second, set by war-time requirements as to definition of fine detail, may be increased to ^ of a second, or even more, where mapping of grosser features is the object. A common, but entirely avoidable limitation, is that due to vibration of the camera. By proper mounting this may be entirely overcome, leaving the ground speed of the plane the only source of exposure-limiting movement. The amount of light reaching the plate constitutes a primary factor in exposure, and this is a matter of lens aperture. Generally, lens aperture is smaller the larger the plate required to be covered, and the greater the focal length. Because of their larger aperture, the short-focus lenses which will be favored for peace-time large-area mapping will permit more and longer working days than have been the rule in long-focus war photography. The necessary use of filters, particularly at the high altitudes which would be chosen in mapping, in order to economize in the number of flights needed to cover a given area, introduces an inevitable decrease in the amount of light available at the plate, as compared with surface photography under the same illuminations.
Broadly speaking, it may be said that all the demands made in reference to aerial photographic exposure work are to decrease the amount of light reaching the plate. Any surplus offered, as by the midsummer noon-day sun, must be immediately snapped up, either by decreasing the exposure to get greater sharpness, or by introducing filters to get greater photographic contrast. The absolute exposure of the plate tends to be kept at the irreducible minimum. As already stated, it lies, with present photographic materials, on the "toe" of the "H & D" curve, just reaching up into the straight line portion.