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This massive enlargement of part of a negative shows the acceptance black line around the letters
Acutance, or edge effects, is something that I have talked about a lot in my book because it is a property that some developers exceed in and many photographers want. I'd like here to discuss some developers that have specialised in acutance.
Specialised acutance developers, such as Crawley's FX-1 and FX-2 are dilute fine grain developers that react mainly with the surface of the film, keeping the image thin and helping to maintain very fine and sharp detail.
An interesting and useful by-product of using a weak developer is how it works on the negative at high contrast edges. What seems to happen is that the developer is locally exhausted by the transition of bromides (the by-products of the faster development on the high density side) across the high density boundary to the low density boundary. At the same time, fresh developer is crossing the boundary from the low density side to the high density side. The high density side keeps locally developing whilst just across the boundary in the low density area the presence of the bromides has slowed development. This increases local contrast at the boundary.
This is strictly a local effect on the boundary line and causes an apparent increase in sharpness. Hence, the reduced agitation used with these developers and others like 510-Pyro and Pyrocat. If the print is looked at carefully, lines can be seen where there is a great contrast change. Along the sharp tops of hills for instance, a light line can be seen where development has been slightly more than over the contrast border.
The developing agent Metol has always been known for its acutance properties and dilute Metol based developers will always create acutance effects; as long as the Sulphite levels are low enough. For instance, using D-23 at 1+3 dilution will improve sharpness (sharper grain structure) as well as improve acutance (these edge effects).
I tested a few good acutance developers for my book and found, as well as D-23, FX-1 and FX-2 that the Pyro/Pyrocatechin developers, Rodinal, and Thornton's Metol two bath were particularly good.
Check it out.