many people are confused about sharpening and when and why to use
it. they don't understand that shooting at sharpness -2 actually
gives them a truer, right off the sensor bitmap image than having
the camera apply sharpening routines.
Sharpening is a destructive process that increases contrast in
certain parts of the image to simulate higher resolution detail.
USM sharpening is used primarily for commercial halftone printing
(newsweek, playboy, artbooks), where overemphasizing the contrast
in certain parts of the image "restores" detail lost in the analog
process of printing and returns the human perception of the image
back to that of sharp photograph when viewed in a book or magazine.
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Images displayed for the web don't need unsharp mask style
sharpening. instead they benefit more from traditional computer
image sharpening, or what i refer to as "edge sharpening"
This is because USM sharpening is a very specific process that is
designed for high resolution images destined for print publication.
These sharpened bitmaps are never really viewed at 100% pixels on a
computer monitor except by the artist preparing the file for
printing.
A properly USM style sharpened image will actually look quite
strange when viewed at 100% pixels. When viewed in a magazine
(which has about 4x the resolution of a computer monitor) the
printed image will look reletively natural. That is to say that the
goal here is to make the final printed image look like the
UNSHARPENED image that the artist was seeing on his computer.
When applying USM to images destined to be viewed on the web at
100%, images can take on a blotchy, smeary effect that actually
softens and degrades the image.
The unsharp mask effect is a trick magazine printers have been
doing for decades which combines two negatives of the same image
(one of which is defocused or "unsharp") to make the image
appear
more natural when viewed in the final magazine. it is an optical
"trick" that can be reproduced digitally by programs such as
photoshop.