The Cine Technician (1953-1956)

Jan.-Feb., 1953 THE CINE-TECHNICIAN 11 and Gevaert have all adopted the same procedure for making duplicates from colour negatives. We think it likely that all the other manufacturers will eventually adopt this procedure, which consists of making silver separation positives, on a panchromatic separation film from the original coour negative. These panchromatic separation films have similar gradation to a normal master positive stock such as 5365. Working this way does not of course remove the spurious images due to the faulty dye transmissions, but it does ensure that no more are added at the intermediate stages of the process. In the case of Eastman negative the defects of the dyes in both the original and colour internegative are compensated by the automatic masking of the coloured couplers. If the complete process of any one of these manufacturers is used the separation positives which are made from the original negative on three silver image films are subsequently printed, with any optical effects incorporated, on to a duplicating colour negative material which can be intercut with the original colour negative if required. Alternatively a complete duplicate negative can be made and the release prints made from that. In fact the possible methods of working are quite numerous and we do not propose to describe them all. In addition to those methods we have discussed above, which use all the colour materials made by one manufacturer in an integrated process, colour negative, especially Eastman colour negative, is being used as original camera material by several laboratories in the United States for printing on to their own release print material, and there is no reason why the same procedure should not be adopted in this country. In such cases as this separations have to be made in order to make the final print, quite irrespective of the need for them for the reasons we have already given. Control Methods In practice the same control methods should be used with colour negative materials as were suggested for stripping monopacks. The register and resolution chart on each roll of stock and at the start of each day's photography, and the lily and pilot scene at the end of each good take. But the most important of all is the transparency step wedge photographed on each roll. The use of the camera as a sensitometer in this way is not quite so simple with colour-developed monopack as it is with stripping monopacks, since the densities given in the three layers have to be read as colour densities through filters. However, with a suitable photo-electric densitometer, individual layer gammas can be plotted and, here again, the correction method to compensate for the uneven illumination given by the camera lens which we have advocated should be used. Eastman state that with a photo-cell with an S-4 sensitivity in the densitometer and the recommended filter packs, the red, green and blue densities as read will be the approximate printing densities for the Eastman Color Print film; but they do not suggest a densitometric technique for their Panchromatic Separation film. This is of some importance and the densities of the three layers as seen by the photo-cell in the densitometer ought at least to approximate to the densities as seen by the separation film through the printing filters. In addition to simplifying the control of individual record contrast, the plotting of the three characteristic curves for the colour negative from the camera step wedge can also be used for the initial grading of the monopack for separation. Since the negative looks a little unfamiliar, visual grading is rather difficult; but so long as the step wedge has been correctly exposed in the standard fashion, the positions of the three curves along the log. exposure axis, relative to a known standard curve position at a mid-density, can be used to assign an overall printer light setting for the whole roll. This does, however, pre-suppose that there are no scenes which are grossly incorrectly exposed. However, in view of the small latitude in exposure of monopacks it is unlikely that such scenes woud be printable in any case. So far no information has been issued for the Gevaert panchromatic separation film but both Ansco and Eastman use the same Eastman stock, 5213, for making the separation positives from their colour negatives and both recommend the same filters for the work; a Wratten 48A 2B for the blue record, Wratten 16 -(-61 for the green and a Wratten 70 for the red. The separations can be made on an optical printer or a contact printer as desired, but all the equipment must have register pins in the correct relative positions. The colour negative is run through the printer three times, once with each of the filter packs, and three black and white positives, thus separating out a red record, a green record and a blue record. The principle difficulty in printer design for this work lies in getting enough light on to the film, for the filter packs, especially the blue, are extremely dense. It is unlikely that speeds better than 15 feet a minute will be possible, in the light of our own experience, even using a 500 watt lamp and a high-efficiency optical system. Since the layer gammas of the negative are known, the gammas to which the three separations must be processed can be calculated, to give a standard overall gamma. The sensitometry is, unfortunately, a little more complex than this statement would lead one to believe because, in the case of Eastman Panchromatic Separation Film at least, the characteristic curve for green light has a much longer toe than it has for red and blue. If it is desired to keep the densities of the separation positive on the straight line then the minimum density of the positive must not be below 0.80. This means that the positives will be distinctly on the heavy side, which may lead to difficulty in printing them, especially if a monopack inter-negative is used in an optical printer as the next stage. If a minimum density lower than 0.80 is used, an average gamma for the portion of the curve used must be taken for purposes of calculation. The recommended developer for the Eastman film is SD-21, which is D.76 with 0.8 gms. of potassium bromide added per litre. After processing, the film still contains a green dye which is added to the emulsion to maintain definition, if a visual densitometer is used with film for control purposes a piece of fixed-out film must be placed in the comparison beam. When these separation positives are used with a build-up type print process the next stage of the procedure is to make straightforward silver duplicate negatives from them in the usual fashion. Although the indirect methods of making separations and the release prints from them may seem somewhat complex and laborious it is likely that, until a stripping monopack is commercially available, they will take their place alongside the direct separation methods in the production of the footage seen in the cinema.