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16-MM CAMERAS AND ACCESSORIES 347
stop-watch. The frame which is located in the aperture when the camera is stopped should then be marked and the film removed from the camera. By counting the number of frames which exist between the two marks on the film and dividing this figure by the number of seconds during which the camera was in motion a very accurate measure of the true film speed will be obtained.
Cameras designed to operate at a number of speeds may be used to obtain 'slow motion' pictures or shots in which the action appears to be speeded up. It must be remembered that, since 'silent' pictures will ultimately be projected at a rate of 16-frames every second, the action can be caused to appear slower than normal by operating the camera at speeds above 16-frames per second. In this manner any given motion is recorded over a greater length of film than would normally occur and, in effect, the time element is stretched. Conversely, operating the camera at speeds below normal will cause the action to appear faster than normal. Under these conditions motion is recorded on a shorter length of film than would normally be used and, when such a film is projected at the usual speed, will therefore appear to take place at a higher speed than recorded at the time the record was made.
It is very important to remember that the time of exposure is controlled by the speed at which the shutter is rotated. The normal camera shutter has an aperture of approximately 170° and, assuming an operating speed of 16-frames per second, will give an exposure of approximately 1 /32nd of a second. However, if the camera is operated at a speed of only 8-frames per second the time of exposure will obviously be increased to 1 /16th of a second. Similarly, if the camera speed is increased to 32-frames per second the time of exposure will be reduced to 1 /64th of a second. It is therefore important to adjust the lens diaphragm when operating the camera at various speeds so that the actual exposure is maintained at a constant value.
A further advantage which may be gained when using a camera in which the speed of operation is adjustable is that scenes which would be under or over exposed when using a simple camera may, under certain conditions, be correctly exposed. Let it be assumed that the lighting conditions are so poor that an adequate exposure cannot be obtained when the lens diaphragm is at its maximum aperture and the camera is operating at 16-frames per second. With a variable speed mechanism the camera may be operated at a reduced speed and the exposure time may be thereby increased. Similarly, if the lighting conditions are such that overexposure is to be expected when the lens diaphragm is at its