Any discussion of high acutance developers should begin with a definition of acutance. Unfortunately this term is not easily explained. However, for the purpose of this article we can think of acutance as the scientific name for what the eye perceives as sharpness. Some years ago, when emulsions were grainier than they are today, developers containing very active silver halide solvents such as thiocyanate and phenylenediamines were popular. Their popularity was not universal since it was found that the reduction in granularity was at the expense of sharpness. As far as the eye is concerned, as it is the final critic in any photographic process, a sharp print is obtained by favoring acutance over grain.
The acutance of an emulsion is related to many variables notably it's thickness, grain size and contrast. Of these the first seems to be the most important. In order to understand this, let us consider a single ray of light from a point source striking two photographic emulsions, a thick one and a thin one. As the ray passes through each emulsion it is scattered by the halide grains. A similar scattering enables us to see the projectors beam of light in a smoky movie theater. If we developed the two films and looked at each image and its cross section they would appear as below. (Fig. 1 omitted) The result of scattering in both cases is to make the image of the point source larger than it should be if the emulsion were infinitely thin. Obviously, the thicker the emulsion the more the image is spread and the fuzzier are its edges. This is why, all other things being equal, a print from a fast film cannot be as sharp as one from a slow film.
In order for a fast emulsion to be fast it must contain more silver halide and therefore must be thicker. Now we can improve on the already considerable sharpness of modern slow speed films by making them thinner. Physically we can't do this but chemically we can, by using a developer which works only on the surface of the emulsion. Such a developer must be very active to offset the loss in sensitivity resulting from using only a portion of the emulsion layer. But in order to prevent excessive contrast the developer must be compensating. That is, it must stop working in regions of high exposure once a certain density is reached while still continuing to work in regions of low exposure. A simple way to solve these two problems is to use a very dilute solution of a very active developer. Because of the high dilution of the developer will be exhausted at the sites of high exposure, keeping the contrast at the proper level. Being very dilute it will have little effect on the silver halides in the interior of the emulsion. In addition high acutance developers usually contain no bromide: the bromide released during development being used to restrain the action in areas of high exposure, thus increasing the compensation.
REFERENCE
Buetler High Acutance Film Developer, Kit No. 01-0060,
Photographers' Formulary
(800-922-5255).
FX-1 High Acutance Film Developer, Kit No. 01-0080,
Photographers' Formulary
(800-922-5255).
FX-2 High Acutance Film Developer, Kit No. 01-0085,
Photographers' Formulary
(800-922-5255).
FX-4 Film Developer,
150 Popular B&W Formulas, Patrick D. Dignan.