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44 PRINCIPLES OF CINEMATOGRAPHY
the camera every second. Assuming, therefore, that the complete cycle of the intermittent mechanism takes place in 1/1 6th of a second, approximately half of this time must be occupied by the mechanism returning to the 'start' position after it has moved the film forward by one picture length. This being so, the actual 'pull-down' must take place within l/32nd of a second. Since the film is required to be absolutely stationary both immediately before and just after the pull-down, the mechanism must, in engineering terms, accelerate the film from rest up to its maximum linear speed within half the period allotted for the pull-down, that is, within l/64th of a second. The distance to be covered by the pull-down is, of course, the height of one picture, that is, 0.750-inch (18.85-mm). A little consideration will show that, at the centre of the pull-down, the film will be moving at a linear speed of 3-feet (91.44-cms) per second. The requirements to be fulfilled by a camera intermittent mechanism are therefore as follows: To accelerate the film from rest up to a speed of 3-feet (91.44-cms) per second and then to immediately retard the speed back to zero the whole process to take place within l/32nd of a second and over a distance of 0.750-inch (18.85-mm).
The foregoing condition only applies to film operating at so called 'silent speed', or 16 frames per second. When operating at 'sound speed', or 24 frames per second, the conditions become even more severe and may be stated as follows: -To accelerate the the film from rest up to a speed of 4-ft 6-inches (137.15-cms) per second and then to immediately retard the speed back to zero, the whole process to take place within l/48th of a second.
Add to the above fact that, under these conditions, the mechanism is expected to position the film to within an accuracy of something less than 0.001 -inch (0.025-mm) and it is not difficult to realise why this section of the camera is considered to be so important. By following the general development of the intermittent mechanism the progress made in general camera design will also become apparent.
When such a course is adopted it becomes difficult to imagine the conditions which existed in the early days of the Industry. The growth of the motion picture has, by comparison with other activities, taken place in an amazingly short period; printing machinery, continuous processing machinery, the many optical devices and, of course, the entire field of sound recording were still to be discovered 50 years ago. In considering these early mechanisms we must, therefore, remember that the pioneers could only judge the quality of their cameras by the resultant