You can find this filter through
->->.
Out-of-focus photographs and most digitized images often need
a sharpness correction. This is due to the digitizing process that
must chop a color continuum up in points with slightly different
colors: elements thinner than sampling frequency will be averaged
into an uniform color. So sharp borders are rendered a little
blurred. The same phenomenon appears when printing color dots on
paper.
The Unsharp Mask filter (what an odd name!) sharpens edges of the
elements without increasing noise or blemish. It is the king of
the sharpen filters.
Some scanners apply a sharpen filter while scanning. It's worth
disabling it so that you keep control on your image.
To prevent color distortion while sharpening, Decompose your
image to HSV and work only on Value. Then Compose the image
to HSV. Go to Image/Mode and click on Decompose. Make sure the
Decompose to Layers box is checked. Choose HSV
and click OK.
You will get a new grey-level image with three layers, one for
Hue, one for Saturation, and one for Value. (Close the original
image so you won't get confused). Select the Value layer and
apply your sharpening to it. When you are done, with that same
layer selected, reverse the process. Go to Image/Mode and click
on Compose. Again choose HSV and click OK. You will get back
your original image except that it will have been sharpened in
the Value component.
