Stereoscopic Aerial Cameras.—Cameras for aerial stereoscopic photography need in no way differ in construction from those made for mapping or spotting, provided only they permit exposures to be made at short enough intervals. The addition of special sights, as already discussed, constitutes the only real difference between single view and stereoscopic aerial cameras. But even without such sights ordinary aerial cameras are applicable to stereo work by the usual procedure of determining the exposure spacing by time.
One scheme employed for taking low stereos, where the interval is only two or three seconds, is to mount two cameras in the plane, exposing them one after the other at the correct interval. Another method which has been tried with success is the use of a double focal-plane shutter in a single lens camera (Fig. [157]). The two shutters are side by side, with their slots parallel to the line of flight. To take a stereo negative we expose first the shutter nearer the tail of the plane, and then the other, after an interval which can be calculated from the speed and altitude, or, better, determined by a stereoscopic sight. The two views are thus obtained on a single plate. Prints from these negatives are transposed right and left, and, if the prints are viewed in an ordinary stereoscope, have to be cut apart and transposed for mounting, or else this may be done to the negatives.
Fig. 157.—Aerial hand camera fitted with two complimentary shutter slits and double sight, for stereoscopic photography.
In this connection attention may be drawn to an alternative method of viewing stereograms, which may be used on transposed prints—a method which needs no instrument, and so has sufficient advantage to even warrant mounting ordinary stereoscopic pairs in the transposed position for observation. This method consists in crossing the optic axes, in the fashion illustrated in Fig. [158]. A finger is held in front of the face in such a position that the left stereogram element and the finger are seen in line by the right eye; the right element and the finger by the left eye. The proper position is found by alternately closing each eye, and advancing or retracting the finger. Then both eyes are opened and converged on the finger tip, which is thereupon dropped, leaving the picture standing out in relief. An opportunity to try this method is afforded by Fig. [159].
Fig. 158.—Method of fusing transposed stereoscopic images by crossing the optic axes.
Stereo Obliques.—The theory of making oblique stereo pictures is identical with that of other stereos. The only problem peculiar to obliques is that of making the exposures at short enough intervals apart. This problem is due largely to the fact that oblique views are ordinarily taken from low altitudes, for the purpose of “spotting” particular objects, rather than for mapping the gross features of an extended area. The same problem of how to secure a short exposure interval is met with when we attempt to take vertical stereos from a low altitude, but as already discussed, it is much preferable from the pictorial standpoint that pictures of definite small objectives be made obliquely.
Another reason for taking stereo obliques from points but little separated is of some interest in connection with the discussion above given of “correct” and “natural” relief. When the relief is “correct” the object appears, as already stated, to be a small model in its true proportions, standing at the convergence distance. When the eyes are converged to a small object 25 to 50 centimeters away all objects beyond are hopelessly transposed and confused. This does not happen when we look at large distant objects, since their background is at a distance effectively but little beyond them. As a result, when a stereo oblique is made in “correct” relief of such an object as the Washington monument with buildings beyond, the confusion of the background presents an appearance entirely contrary to our visual experience with objects as large as the neighboring buildings are known to be. This effect may be avoided by choosing a uniform background such as grass, or by taking the pictures very much closer together, at the expense of “correct” but at a gain in “natural” relief.
Stereo obliques can of course only be made with any facility by laterally pointing cameras. From the calculations already given it appears that a “correct” stereo oblique of an object 500 meters away will mean exposures only two or three seconds apart, too short an interval for any of the ordinary plate-changing and shutter-setting mechanisms; and the case is even worse should less relief be desired. One solution of this problem has been the use, already mentioned, of two cameras mounted together, either side by side or one over the other, with separate shutter releases. Both releases may be controlled by the observer, using a sight, or else pilot and observer may work in harmony as has been recommended in the English service, where the pilot releases one shutter and the observer counts time from the instant he sees the first shutter unwind and releases the second.