[dslater]

(how it works, very roughly: if you make a blurred copy neg from a neg (use an enlarger or stat camera out of focus and 1::1 -- or make two copies, one sharp and one not), then sandwich them together, the areas where they are teh same - no detail - will be black. The areas where the blurred and focused images are different - the details - will let light through. Based on this you can start to use the mark (or its inverse) for burning and dodging.....)

The first time I saw unsharp masking it was a technique for reducing the contrast of astronomical photographs. What you did was to make a thin slightly blurred positive from your negative, then sandwich the negative and positive together and print. What happens is that shadows on the negative are given some extra density by the positive, while highlights are not affected. This reduced the overall contrast of the negative so you can print the highlights down more without making your shadows too dark.

Alternatively, if your negative has a normal contrast range, you can use
unsharp masking for sharpening. Because the contrast of the sandwich is reduced, you can also choose to print on higher contrast paper which increases acutance.