British Kinematography (1950)

198 BRITISH KINEMATOGRAPHY Vol. 17, No. 6 a loop into the gate, within which it is moved intermittently, a second loop feeding it to the second continuous or hold-back sprocket, and thence to the frictionally driven take-up reel. The movement of the film is obturated by means of a rotating shutter. Stroboscopic Effect. The obturating shutter is, of course, the cause of a phenomenon frequently seen even in feature pictures : the effect of wheels or other moving objects appearing to stand still or move backwards. The illusion will be familiar to every engineer as an example of the stroboscopic effect. The relation between film speed and wheel speed may be expressed by the equation : — nR where n = the number of spokes, 7?=speed of the wheel (revolutions per second) and F= speed of film (frames per second). Assuming a shutter opening of 180 degrees, it can be said that : — when p<h movement will appear correct ; when p=h, two sets of stationary spokes will appear ; when p=\, the spokes will appear stationary ; and generally when p— J, \, or any low whole number, there will be some fault in the appearance of the spokes. The precise values and the degree of fault will depend upon the exact shutter opening, and upon such factors as the width of the spokes and the type of lighting. While it would be a very simple matter to make such a calculation prior to the shooting of a scene, actually the focusing system making use of a mirrored shutter permits stroboscopic faults to be observed and avoided. It may, for instance, be possible to alter the speed of the vehicle, or even of the film ; failing either, the vehicle may be fitted with different wheels. The High-speed Camera. A particular form of camera which finds many applications is the high-speed camera, which, of course, produces the illusion of retarded motion, and is, therefore, often known as a slow-motion camera (the German term Zeitlupe, or time magnifier, is particularly apt).6 Although for scientific purposes cameras operating at fantastic speeds up to 100,000,000 images per second have been devised, so far as normal film production is concerned, speeds are rarely needed above 250 frames per second — about ten times normal — and cameras capable of such speeds are of similar type to those just described, with the intermittently moving components lightened for ease of running. While the use of slow-motion photography is quite familiar in news-reels, it may be a cause for surprise that it should find much application in the studio. Its chief application is in model shots. The rate at which a model falls, or the movement of artificially produced waves, will be out of proportion to the scale of a model. The movement is slowed down by the use of the high-speed camera. Whenever a model falls freely a ratio is immediately established between the rate of fall of the model and the rate of fall of the supposedly full-size object, and in theory the reduction in speed should be proportional to the square root of the scale of the model. It must, however, be admitted that experience is. more often used than mathematics to determine camera speed. Process Projection. There are many arguments in favour of bringing the exterior scene into the studio — not the least the uncertainty of British weather. The most widely used method is known as process projection. As far back as 1932, this process