Acutance is something that I have talked about already because it is a property that some Pyro developers exceed in. However, there are some developers that have been deliberately designed to be acutance developers.

Specialized acutance developers are diluted 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 this use of a weak developer is the way it works 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 is strictly a local effect on the boundary and causes an apparent increase in sharpness. Hence, the reduced agitation mentioned earlier. 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.
By the way, Metol has always been known for its acutance properties and dilute Metol based developers will always create acutance effects; as long as the Sulfite levels are low enough. For instance, using D-23 at 1+3 dilution will improve sharpness (sharper grain structure) as well as improve acutance. 
Stacks Image 5