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PRINTING MOTION PICTURE FILM 161
two film rewind heads, and containing a gear reduction unit between two sprockets, one to drive the film and the other to drive the perforated card. The gear reduction unit obviously fixes the speed ratio between the film and the card and must, therefore, be similar to the gearing between the film and card on the printing machine.
If the light-change mechanism is actuated by notches in the film it will only be necessary to place the film in a specially designed punch, where it is located either by guide rails or pins over which it is fitted, the notch then being automatically made to the required depth across the film. A further point which must be taken into consideration is that all relay systems do not respond with equal rapidity. A notch made two perforations ahead of a scene change might cause the light-change to take place at precisely that moment when the scene changed in the printing gate of one particular machine. However, this same notch might cause the light-change to be two or three frames early or late when used in a different printing machine. It should be understood that we are not referring to several machines by one manufacturer, but machines of several different makes, all of which employ the notch method of lightchanging. It is therefore important that the grader should arrange for the notch to occur at the most advantageous point with reference to the actual scene change and, not only should he take into account the lag in the relay system, but also the time required for the diaphragm to reach the required opening.
In the case of grading cards and resistance light-changing, the position of the slots punched across the cards must make allowances for the lag in the relay system and, in this case, for the time taken by the lamp to reach the required intensity level. Obviously, with either system the most severe case will occur when a change is to be made from the lowest available exposure immediately up to the greatest possible exposure. If it can be so arranged that this change is completed as nearly as possible when the actual scenechange passes through the gate, all intermediate changes will be equally accurate and, once the correct relationship between notches in the film edge or slots in the card, and the actual scene-change has been established, this condition will remain unaltered for the particular machine in use.
A parchment card 3-inches (76«2-mm) in width is large enough to accommodate the full range of twenty-one slots across its width and, as we have seen previously, this will cover an adequate range of exposure settings for most negatives. Since twenty-one relay switches are also mounted below the card and across its width, the
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