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310 PRINCIPLES OF CINEMATOGRAPHY
This diagram only illustrates the general principle of rotary contact printing machinery and, in practice, other refinements may be found. For example, one manufacturer includes two further sets of sprocket wheels between 'F,' and the supply spools and also between Ta' and the take-up spools. This is thought to maintain a constant tension on the film spools and, at the same time, to isolate such tension from the sprockets adjacent to the exposing gate. The actual length of the exposing gate, measured along the film, can only be made very small if exceptionally accurate gearing and very constant film speed are maintained. When the aperture size becomes too small any unevenness in film motion will appear as bands of light and dark exposure across the picture area. On the other hand, if the aperture is too large, contact between the two films becomes difficult and definition will become poor.
Most contact printers are provided with means to vary the exposure, either by a resistance light-change or by a diaphragm control, in a similar manner to that found in 35-mm equipment. Since a duplicate 16-mm negative must be made on an intermittent optical reduction printer it is usual to endeavour to complete all the scene balancing operations by adjusting the exposures given when this duplicate is being made.
It should be realised that the photographic quality obtained by contact printing is somewhat different to that obtained by optical reduction printing directly from a 35-mm negative. This is partly due to the fact that optical reduction printers only pass the specular light to the 16-mm film, whilst contact printers pass both the specular and the diffuse illumination to the positive film. The so-called 'Printer-Gamma' in reduction machines is therefore very high. A further reason for differences between the two methods is naturally dependent upon the photographic characteristics of the intermediate 16-mm duplicate negative used in the contact process and, in general, it has been found that slightly higher contrast and, consequently, prints having more brilliance are obtained by the reduction printing process than those possible with the contact methods which use an intermediate 16-mm negative.