The Cine Technician (1953-1956)

II) THE CINE-TECHNICIAN Jan.-Feb., 1953 In an illustration to the paper already mentioned Capstaff showed a rather neat method of obtaining a sensitometric strip between the frames of the work. A nine-step sensitometric tablet is printed in the frame line area by a method which is not disclosed, but is presumably an enclosed light source behind the wedge, which is either just above or just below the gate aperture. This idea is not new although this use of it is, of course, it was in fact patented by Dr. Planskoy in this country a good many years ago. The same effect can be more easily achieved however by the use of a transparency step wedge of a suitable size which will drop into the matte box on any camera. A twelve-step wedge is quite a convenient number since it will give a rectangular tablet three steps by four of about the right proportions. It must obviously be possible to focus the camera on this wedge. In production this grey scale is lit by directing the camera towards a standard white surface at a standard distance, uniformly illuminated to a specified level and at a stop which will give an exposure which is known to be roughly correct. Some ten feet or so of this control wedge is exposed on each magazine loading of film and after the work has been developed the twelve densities are read for each of the three layers. The three gammas can then be measured by plotting the step densities, as read on the film, against the actual densities of the wedge which was used in the camera, marked off along the log. exposure axis from right to left. There is one serious difficulty in using a grey scale in this way however. If the scale is made so that it fills the frame, the film in the gate will not see the densities of the scale in their true relationship, and since we want to use the scale densities as measured on a densitometer, as base points on the log. E axis in later plotting of the three layer gammas of the monopack, we have to do something to correct this. The film does not see the densities in their correct relationship because of the inevitable uneven illumination of the camera lens, no matter how good a lens it is. This lack of illumination at the sides and corners of the frame arises from purely geometrical considerations (Lamberts Lawl and in a lens of average focal length, 50mm., the illumination at the edge of the frame will fall to about 70 per cent of that at the centre of the field and the readings on the film of the wedge step densities at these points will similarly be lower than they ought to be for a correct plot of the gamma. In fact the densities obtained from all steps but those at the centre of the field will be off the true curve, and will make the drawing of the line connecting these points difficult if not impossible. There are two possible ways of attacking this difficulty; the most obvious solution is to make the grey scale so small that it is not seriously influenced by this effect and to place it at the centre of the frame. The difficulty then is that the individual steps will be too small to read on the densitometer, especially important as a photo-electric densitometer has to be used. The second solution, and the one which we think is preferable, enables a full-size scale to be used and consists of modifying the positions of the actual scale densities along the log. E axis when setting out the graphs so as to correct for this uneven lighting in the gate. If each camera has its own scale, as indeed it should, the log. E positions for that camera with that scale can be fixed once and for all by exposing a short length of Plus X in the camera, with the scale in place, to the correct light ing level for this stock. This film is then developed to a known IIB gamma in the usual way. On a sheet of graph paper a line is drawn at this gamma and the densities of steps of the scale on the film are read and plotted on this line. Perpendiculars can then be dropped from these points to the log. E axis. The points so obtained on the log. E axis are then the values to be used for that scale and that lens, whenever the gammas are plotted. In fact they can be permanently marked on a strip of celluloid with an index mark and no actual figures need ever be used. Having calculated the camera gammas from the step wedges on the three negatives the values obtained can then be used to calculate the gammas to which the master positives and duplicate negatives must be processed in order to equalise the contrasts of the three records, in just the same way as is done in black and white work. Colour Negative Monopacks The separations which are made from colour negative monopacks can be classified as indirect separations, and optical methods, that is filtration, are used to make them rather than the physical methods we have so far discussed. It might be wondered why there is any need to separate the three images in a negative of this type at all, since they can obviously be printed directly on to a similar type of positive material which, if it had suitable gradation, could be used as a colour master positive and as many duplicate colour negatives as required made from this. The answer is that the dyes which are generated by a colour developer and couplers are far from perfect and absorb some of the colours which they should pass freely. Magentas and cyans are the worst offenders in this respect, they both absorb blue which they should pass completely and the cyan absorbs some of the green which it should transmit, in addition. The net result of these spurious absorptions is that the colours in prints made from negative monopacks are nearer to grey than they should be. Two lots of this degradation are only just tolerable under the best of conditions, one from the negative and one from the print, but if the duplicating method outlined above were to be put into practice the print would contain four lots of degradation and would be well on the way to being a monochrome result, in addition to sundry other faults. Eastman Color Negative has, as is now generally known, corrections for these spurious absorptions in the case of the negative dyes in the form of coloured couplers and there is no doubt that in the case of a direct print these corrections help the quality of the reproduction a good deal. Generally speaking, direct prints made from an Eastman colour negative are at least as good as prints made by any other process today. However, when duplicates are needed the coloured couplers do not correct for all the spurious absorptions in the materials which would be needed. In addition there is a further factor which is of some importance. One of the most serious problems with any monopack in our opinion, and a problem which is very rarely mentioned, is the difficulty of controlling processing so that the contrasts of the final images in the layers are equal. As we have already explained equal contrasts for the three component images in a colour print is a fundamental requirement for good colour quality. It is for this second reason that Eastman, Ansco