In the first two parts of this series, we
looked at the theory of sharpening and the commands and tool
available in Photoshop. In this installment, we'll look at
some typical (and unusual) sharpening techniques and how to
avoid common problems.
AVOIDING HALOS
Those light and dark halos that sometimes appear when
inappropriate sharpening settings are applied can be
avoided. Decrease the Radius and increase the Amount
setting. Think of the Radius as the width of the "halos" and
the Amount as the degree of sharpening.
WORKING WITH FACES
Sharpening faces can be (but doesn't have to be)
problematic. Getting the correct amount of sharpening to
make eyes jump off the page can also produce skin that looks
aged or diseased. In some cases you need to de-emphasize
wrinkles, too. The key is to sharpen the details of the face
without sharpening the broader areas of skin. The first
example shows (to the right) what can happen when an entire
face is sharpened.
There are a couple of ways to sharpen the
details of the face without creating unseemly detail in the
skin. One technique is, of course, to use the Sharpen tool
rather than a sharpen command, working on areas such as the
eyes, eyebrows, and mouth.
Alternatively, you may be able to sharpen
the Red and/or Green channels to accomplish the same basic
task. Remember that most of the color data for flesh tones
in RGB is carried in the Red channel (the broad, smooth
areas of color), while the Blue channel typically carries
most of the detail (and noise). Here's an example:
The image as a whole has an Unsharp Mask
applied. In the three sections above, you can see the effect
on (from the top) the Red, Green, and Blue channels. Notice
how the sharpening has had a much greater effect on the Blue
channel.
AVOIDING TEXTURED BACKGROUNDS
Buildings or trees against a blue sky, people in front of
walls -- anywhere that a detailed object appears before a
plain surface, sharpening risks adding a texture to that
surface. The sample image from the previous installments in
this series gives a good example. Notice how the sky now has
a pebbled look.
One way to avoid the problem would be, of
course, too make a selection and sharpen everything except
the background. That can be as simple as using the Magic
Wand to select the background and using the command Select>
Inverse. In other cases, sharpening one or two channels
individually can be an easy solution.
SHARPENING CMYK IMAGES
Normally, sharpening should be done before conversion from
RGB to CMYK color modes. However, when an image is received
in CMYK mode and requires sharpening, consider sharpening
only the Black channel. This, in many cases, provides the
improvement necessary without introducing any unnecessary
color shift.
SHARPENING IN L*a*b MODE
Converting an image to L*a*b color mode and sharpening
just the Lightness channel can be a very effective technique
for many images. The Lightness channel carries the can be
considered the Brightness/Contrast part of the image's pixel
information. (The other two channels, a & b, carry the color
information.) Keep in mind that sharpening works by
increasing the contrast between pixels. By sharpening just
the lightness channel, you're working on only that
information. Half of this image was sharpened in RGB mode
(left). The color mode was then changed to L*a*b and the
other half was sharpened, using the same settings, with only
the Lightness channel active.
Notice that the general level of sharpening
is comparable, but there's a slight shift toward red in the
left half (sharpened in RGB mode), and no color shift in the
right half (sharpened in L*a*b mode).
There's another way to sharpen working with
lightness, and you don't even have to convert to L*a*b mode.
(And that makes this technique better for CMYK images.)
• Apply the Unsharp Mask Filter.
• Photoshop 6: Edit> Fade Unsharp Mask.
• Photoshop 5.x: Filter> Fade Unsharp Mask.
• Change the blending mode to Luminosity.
And on the subject of the Fade command,
using it can help take the edge off of a sharpening, so to
speak. In the immortal words of Scott (Mr. Photoshop) Kelby,
"Think of it as Filter-on-a-Slider."
You may also find use for this little trick:
• Apply an Unsharp Mask.
• In the Fade Unsharp Mask dialog box, change the blending
mode to Darken.
• Use the keyboard shortcut to apply the same Unsharp Mask
settings. (Mac: Command-F; PC: Control-F)
• In the Fade Unsharp Mask dialog box, change the blending
mode to Lighten.
Remember that very few images acquired from
a digital camera or a flatbed scanner won't benefit from
judicious use of the Unsharp Mask filter.