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336 iMXEMATOGRAPHIC ANNUAL
The system mostly used today when several prints are needed is to make the negative on standard 35 millimeter film and have recourse to optical reduction printing for the 16 millimeter final copies.
This method is, however, rather costly and the question naturally arises as to why negatives are not usually made on 16 millimeter films, and prints made directly from them by contact printing as it is done for professional film.
There is a quite logical answer to this question.
If we compare the size of the 35 millimeter image with that of the 16 millimeter we immediately realize that in the 16 millimeter film we crowd the same object space in an area approximately one-fourth that of the 35 millimeter film.
It is well known that the photographic image on the film is formed by an innumerable number of silver grains and grain clusters.
It is obvious that graininess controls the resolving power of the film, in other words, its ability to record small details of the object and therefore the smaller the film image the greater is the loss of details.
It can be generally accepted that the greater the speed of the film emulsion the coarser is its grain characteristic and therefore if 16 millimeter film were as rapid as the 35 millimeter the grain characteristic would prove a great disadvantage for the small films when compared with the standard larger size.
Referring to the H & D system of emulsion speed determination, the emulsion with which 35 millimeter is coated has a speed of approximately 750 as compared with 250 to 300 for the reversible emulsion of 16 millimeter films. This difference in speed accounts for the very fine grain of the small film and consequently for its high resolving power.
The slow speed of 16 millimeter emulsions would, however, be rather insufficient to satisfy the public demand if it were not possible to, so to speak, resensitize the film during the reversal processing.
It would be quite out of place in this article to enter into a detailed description and investigation of the factors involved in film processing, but the reversal process can be briefly described as consisting of the following operations.
The latent image resulting from exposure is developed as usual. During this operation, the silver halide affected by the light is transformed in the tiny silver grains and grain clusters which form the negative image. The silver halide of the emulsion which has not been reacted upon by the developer is still sensitive to light. It is obvious that if the silver particles forming the first, or negative, image are bleached and the remaining silver halide is exposed to an actinic light, a latent image is formed which, if developed through the normal developing process, will form a photographic image which will be positive.
It is through an accurate control of this double development and intermediate "flashing" (exposing to light) that the sensitivity "boosting" effect is secured without increase, in fact with a reduction of graininess.