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330 CINEMATOGRAPHIC ANNUAL
Motion picture film, be it of 35 millimeter or 16 millimeter size, is, during projection, subjected to a great magnification and therefore any mechanical imperfection in the construction of the machines through which it is made to pass (perforator, printer, camera or projector) is many folds multiple upon the screen.
The average magnification to which 35 millimeter film is subjected during projection on a theatre screen is approximately from 350 to 400 times diametrically.
An error of registration of subsequent picture frames of one thousandth of an inch results in a quivering of the screen image for an extent nearing half an inch, which is very noticeable under normal conditions and cannot be tolerated in cases where the unsteadiness of the projected image is made more apparent by means of comparison. Titles, for example, demand great accuracy of register because of easy comparison with the stationary border of the screen and obviously any picture which has involved a double or multiple exposure would be utterly spoiled if the records at different times were not registered with the most exact accuracy.
Under all conditions, an error of register of one hundredth of an inch would result in a quivering of the projected image to the extent of four inches, which is absolutely intolerable.
It is true that the magnification usually exacted from 1 6 millimeter film is not as great as that to which 35 millimeter films are subjected, but it is also true that the distance from which 1 6 millimeter projection is seen is usually less than that of 35 millimeter and therefore, errors of registration and unsteadiness of the image are as visible and as disturbing.
Perfect film registration involves mechanical problems of difficult solution, mainly because of the shrinkage characteristic of the film. Both 35 millimeter and 16 millimeter films consist of a thin layer of gelatin which serves as a binder, so to speak, for the extremely minute particles of a silver compound, (which is the light sensitive material) which is in turn coated over a celluloid support, called the base.
This base is an unstable material. That is to say, it is subject to swelling and shrinkage according to the treatment it receives, and also according to the atmospheric conditions in which it is placed.
Film stretches during the photographic wet processes of development, fixing and washing, and shrinks while drying.
After it is dry and the shrinkage effect continues, to a lesser or greater extent according to the atmospheric conditions in which it is placed.
The manner in which the finished film is stored, the frequency of projection, and atmospheric conditions are all causes which provoke constant changes in the length and width of a strip of film, and these changes cannot be controlled with absolute certainty regardless of the care with which film may be treated.
It is quite obvious that under these circumstances it is not possible to devise machinery which will automatically compensate for differences of dimensions due to film shrinkage. The motion picture mechanical engineer had to devise means to accommodate these differ