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272 CINEMATOGRAPHIC ANNUAL
of the motion picture world the basic principles upon which are based the whole of the technical structure of motion picture production.
It would seem at first thought, that once standard dimensions as to size of film, size and spacing of perforation, and dimension of picture frame are established, the task of conceiving and constructing apparatus to handle it throughout its processing, should not present other difficulties than those inherent to accuracy of design and manufacture, and such would indeed be the case if motion picture film was a stable material.
Unfortunately, however, the celluloid base of films is subject to stretching and shrinkage, according to its moisture content, and to the atmospheric conditions in which it is placed.
This stretching or shrinking of the film is, furthermore, beyond human control and even when films are manipulated with the most exact care, the changes in dimensions which result from these phenomena offer a multitude of serious problems, the solution of which has been the goal of the motion picture mechanical engineer.
If we consider a roll of film at the moment that it leaves the manufacturing plant, we can consider it as being cut and perforated to the exact dimensions which have been accepted as standard.
This roll is carefully packed in proper wrapping material and in sealed cans under ideal conditions of temperature and humidity, and it is reasonable to expect that under such conditions, its dimensions will remain unaltered until the time that it is withdrawn from its container for use. Then the following conditions are to be contended with:
1. Shrinkage occurring from dryness from the time that the film is withdrawn from its container and the time that it is run through either camera, printer, and developing solutions.
This shrinkage effect can in practice be disregarded for positive film because it never leaves the laboratory rooms where atmospheric conditions are kept under control.
Negative film may shrink sufficiently to create some trouble, especially if left in the camera magazines for a length of time or used under conditions of extreme heat, cold or high degree of humidity.
2. Swelling and stretching during the wet processes of developing, fixing, washing.
3. Severe shrinkage during the process of drying.
4. Continued shrinkage after the film is completely processed.
Positive films are obviously more liable to be seriously affected by shrinkage because of their long running through projection machines and their constant exposure to variable atmospheric conditions, while negative films are stored away with special and utmost care, and their shrinking takes place mostly during the relatively short time that they are run through the printing machine.