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PRINCIPLES OF CINEMATOGRAPHY
Fig. 152. — Spring-loaded oscillating claw mechanism.
effects, it is necessary that the camera shall be fitted with means to rewind the film. On first consideration it may be thought that an auxiliary handle, fitted to an extended gear-shaft, would be all that was needed to provide this refinement. Whilst this may be true in some cases it can only operate successfully under certain conditions. Some cameras are fitted with intermittent claw mechanisms which do not move out of engagement from the film perforations at the end of the pull-down stroke but merely oscillate in a vertical manner. Such a mechanism is seen in Figure 152 and it will be realised that, by spring loading the top of the claw lever, this section can be caused to automatically 'fold back' as the lever commences to move upwards.
As shown in position 'A' the claw has just commenced to pull the film downwards in the direction of the arrow and, since the hinged tip 'D' is spring loaded at 'E', a small projection 'F is caused to engage with the main body of the lever 'G' and so force the mechanism to act as if it were one solid piece of metal. After the film has been drawn downwards and the claw commences to move in an upward direction, as shown at 'B', the small nosesection will hinge about point 'H' and so cause the claw tip to disengage from the perforation. This will cause spring 'E' to be compressed still further and, unless the tension in the film gate is greater than the force exerted by this spring, the film may move upwards during this part of the cycle .After the claw tip has moved some distance in an upward direction the relationship between it and the film will be as shown in diagram 'C, Figure 152.
It will therefore be apparent that a mechanism of this type cannot be used in conjunction with a film rewind unless the operator is prepared to remove the film from the gate whilst