Most of digitized images need correction of sharpness. This is due to
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 Sharpen filter accentuates edges but also any noise or blemish and
it may create noise in graduated color areas like the sky or a water
surface. It competes with the Unsharp Mask filter, which is more
sophisticated and renders more natural results.
