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116 PRINCIPLES OF CINEMATOGRAPHY
grating exposure meters, exceptionally small areas may be measured and precise exposure on each part of the scene can be forecast.
The next point which must be covered adequately if photographic consistency is to be maintained is, naturally, that the carefully exposed film shall be processed in the laboratory so that the anticipated result is achieved. This requires that the laboratory shall be able to measure the activity of their processing solutions, be able to control the time during which the film will remain in these solutions, and keep the solution temperatures within close limits. Since laboratories receive many different types of film from different studios and, indeed, films of different emulsion speeds may be received from different production units within any one studio, it is necessary for the laboratory to have some means of testing the speed of these films before the studio negatives themselves are processed. Clearly, their equipment must be sufficiently flexible to permit adjustments to the developing technique in order to obtain the photographic conditions favoured by any particular customer. This change is not normally very big in picture negatives but, in sound recording, considerable differences in negative contrast are required by some types of recording and will depend largely upon the equipment with which the prints from the original sound negatives are to be used.
At this point it will be helpful to remember the many intermediate steps between the original negatives shot on the studio floor and the final release positive prints as seen in the cinemas, (see Chapter I).
It should now be apparent from the foregoing brief survey that film may be processed under a variety of conditions and that, in fact, continuous processing machines should be designed to provide the greatest possible latitude in this respect. It should therefore be obvious that, to harness this latitude to produce the optimum results when any one type of film is being processed, some form of control must be employed to correlate the characteristics of the film emulsion with those of the developing solutions and machine conditions.
For these reasons a system known as 'Sensitometric Control' has been applied to continuous film processing although, as will become apparent, such a system may be applied to any monochrome photographic process. Broadly speaking, sensitometry is the study of the following characteristics of sensitive emulsions and processing techniques : — (a) The photographic speed of the emulsion, (b) The Gamma or contrast which may be obtained with a given emulsion under known processing conditions, (c) The