Fig. 129.—Diagram giving data on area covered at various altitudes by representative lens.
Fig. 130.—Burchall Slide Rule, for calculating intervals between exposures, and for other aerial photographic data.
In considering the question of sights, attention may again be called to the poor “visibility” from the pilot's seat in the present prevailing type of two-seater tractor plane. Blind directly in front, beneath, and to either side (Figs. [7], [8] and [9]), it is no unusual thing for a pilot to entirely miss an objective, such as a railway line, which he can only estimate to be beneath him by judging its distance from those objects to either side which he can actually see. The English practice of leaving a clear space of six inches to a foot between the fuselage and the beginning of the wing fabric, allows the pilot to look down over the side, a decided advantage. But for photographic purposes nothing can compare with a good negative lens carrying fore and aft lines or wires, so that the pilot can see his objective in ample time to head directly for it. The lens should either be large enough so that its rear edge gives the view directly downward, or supplemented by an additional lens pointing directly down, so that the covering of the target is assured. To locate such a lens in the front cockpit, free of all controls, is a very hard task; even so its view is likely to be badly interrupted by the landing gear. Nevertheless, so important is it, both in photography and in bombing, to have a sight by which the plane can be accurately directed that designers of planes should recognize this need and make every effort to provide a suitable location.
Fig. 131.—Aerial bombardment of Trieste. Note falling bombs in center of picture; and exploding anti-aircraft shells over the water.
Italian official photograph.
Fig. 132.—Example of spotting requiring exposure at exact instant. Explosion following burst of bomb in ammunition dump.
British official photograph.
Sights for the observer have been discussed already. Here again the negative lens is to be preferred, but while the pilot's lens needs only directing lines in the axis of the plane (unless he takes the picture), the observer's lens needs both an accurate center mark and an additional upper or lower sighting point. Accurate alignment of these marks with the camera axis must be arranged for in precise spotting.