British Kinematography (1948)

39 amateur laboratory it is practically impossible to take the many expensive precautions required to avoid them. However, as these dust marks originate only when copying, they are totally absent from a reversal film, unless they originated at the moment of shooting, due to dirt on the film or in the camera gate. The use of the reversal process for certain purposes in professional kinematography could render great, services, and the more complicated manipulation would be amply rewarded by the elimination of a copying process as by the use of the reversal process for making dupes. Incidentally, it may be pointed out that in colour-films each copying step results furthermore in a degradation of the colours, so that, especially in the case of colour films, the elimination of a copying step is of prime importance. Important Drawbacks Against these advantages of the reversal process versus the negative I positive process there are, naturally, also important drawbacks. The first ; one lies in the more complicated development process, and the fact that the ordinary developing machines are not suited for this kind of work. The ; process could eventually be carried out on ordinary developing machines ! provided it is carried out in two steps, as on two different machines. As by the use of the reversal process, for example in duplicating, one reversal film replaces two other films, the operation would not be so much more expensive than with the ordinary process, whilst, furthermore, the advantages would still be retained resulting from the elimination of a copying step. The principal disadvantage of the reversal process, however, is a lack of latitude in exposure. In principle, shots on a reversal film do not allow a latitude of exposure if the development is carried out in a standard method, although there are various ways of saving a reversal film that has been wrongly exposed. Latitude of exposure is of no importance when using reversal film : as an intermediate material in the laboratory, where the exposure can be precisely regulated. Neither has positive film by a standard development any latitude of exposure. li. REVERSAL PROCESSING METHODS The method in which the reversal process is carried out may be summarised as follows. The reversal film which has been exposed, either in the camera or in the printer, is first thoroughly developed in a vigorous developer. By such development it must be understood that the emulsion within the area of the total exposure is developed to the highest density attainable, that is to say, that in this area, after developing, absolutely no reducible silver halide remains. In practice this can only be attained by adding a solvent of silver halide to the developer. This development naturally results in a somewhat strong fog, and the presence of a solvent of silver halide leads moreover to the formation of a dichroic fog, so that ultimately the sensitometric curve, of the film after such a first development appears as is illustrated by Fig. 1. After this first development usually follows a stop-bath, which at the same time acts as a hardener. This hardening is in most cases necessary because the lengthy manipulation required by the reversal process would otherwise occasion a softening of the gelatine layer. As a stop-bath, the well-known solution of 3% chrome alum may be used. After the stop-bath the film is thoroughly washed. It is indeed imperative that all the developing products be eliminated, in view of the danger that, lue to oxidation in the following bleaching bath, these may create a brown stain which cannot be removed. * I