If we define the _efficiency_ of a shutter as the ratio of the amount of light it transmits during the exposure to the amount of light it would transmit were it wide open during the whole period, then the efficiency of the ordinary between-the-lens shutter is of the order of 60 per cent. This means 1.6 times the motion of the image for the same photographic action that we should have with a perfect shutter. The accompanying photographic record (Fig. 20) of the opening and closing process of this type of shutter clearly illustrates its deficiencies. [Illustration: FIG. 20.—Effective lens opening at equal intervals of time: (_a_) during focal plane shutter exposure; (_b_) during between-the-lens shutter exposure.] =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. [Illustration: 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_