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THE CINEMATOGRAPH CAMERA 61
the claw pins. It is important to note that the nose of the pilot pins projects sufficiently to overlap the nose of the claw pins, and, because of this, the film is always under control by one or both sets of pins. By this arrangement it is possible to dispense with any tensioning in the gate channel, except for that which is automatically applied when the gate closes completely on to the pilot pins. The claw pins now move in a downward direction, as seen at *D\ and so advance the film by an amount equal to one frame or picture. The gate then moves to the left, as seen at 'E', and transfers the film from the claw pins back to the pilot pins. The cycle is then repeated.
The pilot pins are separated by a distance equal to five 'pitches', as the distance between each perforation is termed. It is because of this control over a considerable length of film, and also due to the accuracy with which the pilot pins fit the perforations, that it is possible to dispense with the conventional edge guiding in the film gate. The film is therefore freely suspended on the pilot or claw pins except for that part of the cycle during which the gate is closed and the film comes under pressure from the aperture plate.
It should be noted that the pilot pins are of different sizes. Pin No. 1, is known as the full-fitting pilot and is manufactured accurately to fit the perforation in the direction parallel to the film length. Pin No. 2, is known as the tolerance pilot and is suitably reduced in thickness to accommodate the permissable tolerances on film length over a section five perforations in length. At the moment when the claw has completed its downward stroke, and is registering a perforation with the full-fitting pilot pin No. 1, the relationship between the pilot pins, claws and the punches in the perforating machine will be as follows: The punch in the perforating machine which pierced the hole engaged by the claw pin will also have pierced the hole into which the pilot pin will move. It will be remembered that this condition is required to ensure maximum protection from the effects of any errors which may have occurred in the perforating of the film.
Variable Aperture Shutters
The normal opening in a rotary shutter blade is approximately 170°, or just under half the complete revolution. Quite obviously any alteration from the selected opening would cause a change in the amount of exposure given to the film. The ability to control the exposure by this means is a decided advantage for two main reasons. Firstly, from our knowledge of still camera practice, it is well known that a lens working at high aperture, and critically