As a percentage error this one per cent. is small compared with other uncertainties, such as film shrinkage or the error of level of the camera. As an absolute error in surveying, thirty feet is, of course, excessive. The distortion is diminished for any specified shutter speed by making the speed of travel of the curtain as large as possible and by correspondingly increasing the aperture. In connection with film cameras, another solution which has been suggested is to move the film continuously during the exposure in the direction of the plane's motion. The requisite speed of the film _v'_ to eliminate distortion is given by the relation: _v'_ _F_ ———— = ——— _V_ _A_ For the values of _V_, _F_, and _A_ used above, _v'_ = .92 centimeters per second. This speed is clearly that which holds the image stationary on the film—a fact which suggests another object for such movement, namely, to permit of longer exposures. The effect of focal plane distortion may be averaged out in the making of strip maps, if the shutter is constructed so as to move in opposite directions on successive exposures. The first picture will be magnified, the second compressed, and so on, but a strip formed of accurately juxtaposed pictures will be substantially accurate in over-all length. Such a shutter is embodied in one of the German film cameras (Fig. 61). Distortion of the kind above discussed is absent with between-the-lens shutters, which may conceivably be improved in efficiency and in feasible size. If so they would merit serious consideration for aerial mapping. =Methods and Apparatus for Testing Shutter Performance.=—With a focal-plane shutter the desirable qualities in performance are three in number: (1) _Adequate speed range_, which may be taken as from 1/50 to 1/500 second for aerial work, (2) _good efficiency_, which has already been treated, and (3) _uniformity of speed_ during its travel across the plate. Before the advent of aerial photography little attention was paid to speed uniformity, differences of 50 per cent. in initial and final speed being common in focal-plane shutters, and but little noticed in ordinary landscape work because of the natural variation of brightness from sky to ground. In the making of aerial mosaic maps the non-uniformity of density across the plate results in a most offensive series of abrupt changes of tone at the junction points of the successive prints (Fig. 140), an effect which must be minimized by manipulation of the printing light.