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Chapter 15 Digital Darkroom Basics
can improve one that is slightly soft. The most
common use for the sharpening technique,
however, is overcoming the slight blurring that
can occur when an image is resized, rotated, or
otherwise processed digitally.
Sharpening is actually an optical illusion:
it does not restore lost detail, but increases
the difference in the color of adjacent pixels,
especially along edges. This tricks the eye
into seeing the image as more detailed and
sharp. The technique must be used with
care; oversharpening can give the image an
unattractive, harsh, blotchy appearance. See
Figure 15-45.
An image editing program may offer
several sharpening tools or filters. Photoshop,
for example, lists five choices: Sharpen,
Sharpen
Edges,
Sharpen
More,
Smart
Sharpen, and
Unsharpen
Mask. Although it has the least-likely
sounding name, the most often-used of these
tools—because of the control it offers—is
Unsharp
Mask. Three of the remaining choices
are “all-or-nothing” in their approach, providing
a set amount of sharpening (minimal in the case
of
Sharpen
and
Sharpen
Edges, a considerably
greater amount in
Sharpen
More). The fifth
choice,
Smart
Sharpen, has the same control
advantage as
Unsharp
Mask, plus some additional
features that make it somewhat more difficult
to use.
The
Unsharp Mask
filter creates a slightly
blurred copy of the image and uses it as a
mask (hence, the term unsharp) to determine
which areas will be sharpened. As shown in
Figure 15-46, the degree of sharpening can be
controlled and previewed when using this filter.
The chosen degree of sharpening is displayed
on both the full-size screen image (for accurate
judgment of the effect, the displayed image
should be at 100 percent) and on a small detail
view. By clicking on this detail view, or by
checking and unchecking the
Preview
box, you
can compare the image before sharpening and
with sharpening applied.
Figure 15-45. Oversharpening can seriously degrade image quality. A—A properly sharpened image. B—The same
image that has been badly oversharpened.
A B