skoro som odpadol ked som si toto precital:
Unsharp masking is another very popular image enhancement technique.
From a mathematical point of view its justification is a bit obscure but many people are very fond of it. The first step is to blur the source image, hence the name unsharp masking. Then the difference between the source image and the blurred image is subtracted from the source image.

REFOCUS plugin to gimp:
http://refocus.sourceforge.net/doc.html

idem skusit.. :)