Characteristics of the Focal-Plane Shutter.—Long before the days of aerial photography the problem of a high-efficiency high-speed shutter for photographing moving objects on the ground—railway trains or racing automobiles—had already led to the development of the focal-plane shutter. This is a type peculiarly adapted to the problems of the airplane camera. It consists essentially of a curtain, running at high speed close to the photographic plate, the exposure being given by a narrow rectangular slot.
If the focal-plane shutter is in virtual contact with the sensitive surface the efficiency, as defined above, is 100 per cent., since the whole cone of rays from the lens illuminates the plate during the whole time of exposure. But if the curtain is not carried close to the plate the efficiency falls off rapidly with distance, especially so for small apertures of the slot.
Fig. 21.—Calculation of focal plane shutter efficiency.
The efficiency of the focal-plane shutter may be calculated as follows: Let the focal length of the lens be F, its diameter be F
N, the width of the slot be a, and the distance from plate to curtain d (Fig. [21]). Now if the curtain is moving at a uniform speed, the time taken for the slot to traverse the whole cone of rays, from the instant it enters till the instant it leaves, will be directly proportional to
| d | (F) | d | ||
| (—) | + a = | + a | ||
| F | (N) | N | ||
If the curtain were in contact with the plate the time taken for the same amount of light to reach the sensitive surface would be proportional to a. Again defining shutter efficiency as the ratio of the light transmitted to what would have been transmitted were the shutter fully open for the total time of exposure, the efficiency, E, is given at once by the expression—
| a | ||
| E = | ||
| d N | + a | |
As an example let the lens aperture be F/6, so that N = 6; let d = 1, and a = 1, then E = 6
7. In the French deMaria cameras, where d = 4 centimeters, E = 60 per cent. for the aperture assumed, which is representative. Fig. [22] exhibits diagrammatically the chief characteristics of the focal plane shutter.
