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28 PRINCIPLES OF CINEMATOGRAPHY
sprocket teeth. The film was intermittently advanced through the gate by a simple link mechanism, known as a claw, the operation of which caused the claw tips first to engage with the film perforations, pull the film downwards by a distance equal to one picture length and then retreat away from the film and travel upwards in readiness to repeat the operation. This action is clearly seen in the series of pictures shown beside the camera in Figure 4.
Geared to the claw mechanism, and rotating at right angles to it, was a disc shutter, mounted so as to interrupt the light passing from the lens to the film. The shutter consisted of a flat plate having an aperture cut away to enable light to pass only when that section of the disc was between the lens and the film. This shutter is also shown beside the camera in Figure 4 and at the various positions it would occupy relative to the claw mechanism.
Thus the action of the camera was as follows : Firstly, the shutter would pass across the lens and so stop the light from reaching the film. The claw tips would then engage with the film perforations at the top of their stroke. This condition is seen in Figure 4 at 'A'. The claw would then proceed to move the film downwards by an amount equal to one picture length, as seen at *B\ Figure 4. The claw would then disengage from the film perforations and the shutter open as seen at 'C, Figure 4.
From this moment, until the claw and shutter again reached the positions shown at 'A', the film would remain stationary in the gate and would be receiving an exposure. For the sake of completeness a further condition of the claw and shutter have been shown at 'D', Figure 4, in which the claw has returned halfway along its path back to the top of the cycle and the shutter has completed half the exposure.
From the foregoing it will be realised that the film will move through the gate by a series of intermittent forward motions, interspersed by a series of rest periods. However, for other reasons, it is necessary to maintain a constant forward motion of the film at the central sprocket. Because of this, loops are provided above and below the gate and serve to absorb the relative change in length of the film which takes place whilst the camera is running. Thus the loops are continually varying in length, the top loop being relatively longer when the lower loop is short, and vice versa. Passing from the central sprocket the film is led via a light-trap into a spool box, similar to that containing the unexposed film, where it is automatically wound onto a central bobbin.
A description of the Prestwich camera has only been included