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MOTION PICTURE PHOTOGRAPHY
have as wide a shutter opening as the harmonic cam. Some of them decrease the time in which the film is moved down by having a crank whose throw is greater than the distance from picture to picture, and use only a portion of the crank throw for drawing the film down, the engagement of the pins or claws with the film taking place after the crank has commenced to move downward and releasing before the crank reaches the bottom of its throw.
There are many variations of the rod and crank movement. In the Pittman model the fingers are upon springs actuated by a crank. The fingers move in a circular path except when drawing down the film, where they are forced to subtend a chord of the circle by a friction plate in the plane through which the film moves. This friction plate being struck i6 times per second by the revolving spring claws makes this movement a very noisy one. In the Williamson movement a small arc-shaped slot guides the pins in an approximately straight line during the period of their engagement with the film. In other movements a double crank is used, giving both the in and out and up and down movement to the claws. A third variety of movement which was much used a few years ago was called the slip claw movement. In this movement the claws were ratchet-shaped and in their upward travel slipped along the perforation as a pawl slips over a ratchet.
The Pathe Freres formerly made an amateur model which used the slip claw movement. The slip claw movement has almost entirely gone out of use because it could not be reversed. No matter in which direction the crank of the camera was turned the slip claws would pull the film down in the same direction. An inadvertent throwing back of the crank, for even a fraction of a revolution, would cause the film to lose its upper loop. Unless there was a great nicety of adjustment between the friction at the gate and the pressure of the spring claws they were liable to push the film backward on their upward travel, causing the frames to overlap, thereby making what is called a creep in the film.
The in and out movement, or the movement of the pins in and out of the perforations, is accomplished in various cameras by many different methods. A positive in and out movement is much to be preferred over one which is accomplished by some
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