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. [Illustration: FIG. 22.—Characteristics of focal plane shutter.] In view of the necessity for _some_ distance between shutter and plate it is obviously important to keep _a_ as large as possible, depending for the requisite shutter speed on the velocity of the curtain. Large aperture and high curtain speed are also found to be desirable when we consider the distortion produced by the focal-plane shutter. =Distortions Produced by the Focal-plane Shutter.=—While the time of exposure of any point on the plate can, with the focal-plane shutter, easily be made 1/100 second or less, the whole period during which the shutter is moving is much greater than this. For instance, a 1 centimeter opening which gives 1/100 second exposure takes ⅒ second to move across a 10 centimeter plate, or nearly ⅕ second for an 18 centimeter plate. With a moving airplane this means that the point of view at the end of the exposure has moved forward compared to that at the beginning, by the amount of motion of the plane in the interval. If the shutter moves in the direction of motion of the plane the image will be magnified; if in the opposite direction, it will be compressed along the axis of motion. The amount of this distortion is calculated as follows: Let the velocity of the plane be _V_, and that of the shutter be _v_. Let the focal length of the camera be _F_, and the altitude _A_. If the camera were stationary, a plate of length _l_ would receive on its surface an image corresponding to a distance _A/F_ × _l_ on the ground. Due to the motion of the shutter the end of the exposure occurs at a time _l/v_ after the start. In this time the plane has moved a distance _V_ × _l/v_; hence the point photographed at the end of the shutter travel is _Vl/v_ within or beyond the original space covered by the plate, depending on the direction of motion of the curtain. The distortion, _D_, is given by the ratio of this distance to the length corresponding to the normal stationary field of view: _V/v_ × _l_ _VF_ _D_ = ——————————— = ———— _A/F_ × _l_ _vA_ When _V_ = 200 kilometers per hour, _v_ = 100 centimeters per second, _F_ = 50 centimeters, _A_ = 3000 meters, we have— 20,000,000 × 50 1 _D_ = ———————————————————— = approximately ——— 3600 × 100 × 300,000 100 Or if the actual distance error on the ground is desired, _Vl_ ———— = 10.8 meters _v_