Graphics Reference
In-Depth Information
Only sharpen images that really need it, and then use the least amount of
sharpening to get your result. Avoid the standard Sharpen filter: It's a blunt instru-
ment for a surgical task.
The correct image sharpening tool is the “Unsharp Mask” filter, one of the
Sharpen menu's alternatives. It only affects areas where abrupt color or value changes
indicate visual edges. The filter's strange name describes how it works. It takes a
blurred copy of the image and uses it as a pixel mask, then compares each pixel with
its blurred version. Pixels that aren't at the edges are untouched, or masked out.
The filter then does some complicated math to determine how much to change the
edge pixels.
If you have images that develop outlines when you use Unsharp Mask, try
working on each image channel separately, sharpening the noisiest channel (usually
blue) the least, and the cleanest channel (usually green) the most. But be careful.
Sharpening one channel much more than the others can lead to color shifts.
Make sharpening the last thing you do before you save a file in a compressed
format. As with other edits, never sharpen a file that is already compressed. Return to
a universal file format and make your changes there, then resave the file for the web.
Unsharp Mask's Radius and Amount
should increase with your resolution.
At 72 ppi, Radius should be between
.3 and 1.0 pixels, with .5 to .8 usually
giving optimal results without over-
sharpening. For Amount, I start at a
default of 75% and move up to 175%,
watching for harsh lines that warn me
that I've gone too far. Threshold
should decrease as resolution
increases. A Threshold of 3 to 5 is
appropriate at screen resolution.
