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By C.W. GIBBS, A.R.P.S.
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Though the theory and practice of the reversal process is of great interest to the filmmaker it is usually a great mystery to her. Primarily this is because it is thought that special equipment is necessary for processing the long lengths of film. For under a roll or two, however, this is simply untrue. Short lengths of two or three feet can be processed by standing the film on edge in the form a coil in one of the usual photographic trays. Some experimenters wind a short length of film around a piece of glass and process it in a tray; others build small racks or drums and set up a regular cine laboratory. Commercial apparatus is available for use in this work. But no matter what method is utilized it is fun to do your own work.A brief summary of the reversal process is--first the film is developed, instead of being placed in a hypo bath (fix) to remove the unexposed silver halide it is placed in a bleach bath which removes the developed grains. The unexposed grains remain in the emulsion so it is still light sensitive. The film is exposed to white light, developed again, fixed, washed and dried. Naturally this all sounds simple when you say it in a few words like that and we know the logical question is "Yes, but how does the film become a positive?"To answer this we have to say things in a manner which is not exactly scientifically correct by using an illustration. Consider on the film you have three areas, one very light, one dark and the third medium grey. The light area represents a shadow tone, the dark area a highlight and the medium grey the half-tones. The highlight area of course develops to a high density because the intense light in that area of the original subject affected almost all of the grains in the emulsion. In the shadows but little light was received so only a few or none of the grains at that spot were affected. In the half-tone portion we will say in this case that fifty per cent of the grains were affected by the light.As we put the film through the first developer it develops up to a negative image in the normal manner. If the film at this point was placed in a hypo bath (fixer) to dissolve out the unaffected grains we would have a negative with dark areas representing the highlights and light areas representing the shadows.In the reversal process the film is not fixed at this point but instead is placed in a bleach bath. This bleach bath removes the black silver image, leaving the unexposed crystals. The image on the film is now composed of unexposed crystals--many crystals in the shadow areas and none in the highlight area. In the half-tone area, fifty percent remain. Many crystals are present in the shadow area because only a few were affected by the light and by the first developer. In the highlight area where the first developer produced a heavy density all the crystals were used up and after bleach none remain. Thus if the film was now exposed to white light the shadows would have a very heavy density if the film was again developed.The film is really fogged but it produces an image on the film wherever there is a lot of silver present in the emulsion. Where there is little silver left, there is little density; a lot of silver, a heavy density.Before passing on to actually taking up the processing in detail there is another bit of theory which we must explain about the silver grains in an emulsion. There are present in every emulsion numerous grains which vary considerably in size. Grains of different sizes have different characteristics. For example, the larger grains are faster. In emulsions of various types there are what we usually call varying grain size distributions. In a fast negative emulsion there is a high percentage of large fast grains. In slow emulsions there are but few large grains. Emulsions manufactured for use in the reversal process have the grain sizes very carefully designed to fit the procedure. That is, there are a number of large fast grains to form the image in the first developer and also there are a large number of small insensitive grains which are to form the positive image.Many experimenters endeavor to reverse regular fast negative emulsions and have but little success because there is too small a percentage of small grains to in the emulsion to form a positive of good quality. More success will be had if a positive or process emulsion is used for reversal experiments because general characteristics more nearly approximate the reversal type. If a positive or process emulsion is to be reversed the standard formula given later for use will be found too contrasty. A soft working fine grain developer will be more suitable with those emulsions. |
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Taking up the actual processing of the reversal emulsion we are giving formulae which the writer has used for a long time and have given very satisfactory results. There are a number of formulae given for reversal and if the experimenter wishes to change one of them--he will find himself getting into difficulties, by that we mean if she selects a first developer from one series of formulae, a bleach from another series and a second developer from still another series she will find that the results are not quite what she would like to have. Start by sticking to one series of emulsions. (Editor's note: the series given here is rather advanced [it involves mixing some rather toxic chemistry], and shouldn't be attempted unless you know what you're doing. For the first timers, pick up a T-max direct positive kit. This process is given in the interest of preserving old formulae and encouraging experimentation. Enjoy, but be careful.)The exposed reversal film is developed in the following developer for five minutes.
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The troubles which might come are stains and poor image color. These results are from an exhausted bleach bath or old first developer. Too short a first development will also result in poor image color. If the positive is too dark the film was either underexposed or first development was too short. If the positive is too light it was due to being overexposed.
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