| Many images, particularly
scanned ones, can benefit
from a bit of sharpening.
Unsharp Mask (USM) is
usually a good choice,
but when used at higher
settings it can introduce
unwanted color shifts.
This is because the same
tonal edges can vary in
each channel. If an USM
is applied to all channels
equally, it can affect one
channel more than another,
distorting the color. Instead, use
Photoshop’s Lab Color
mode to sharpen an image
without shifting any of its
colors. For the uninitiated,
Lab Color mode separates
the image detail from the
image color. It was created
to be device-independent,
and Photoshop actually
uses it quite a bit behind
the scenes—even when
you’re in another mode.
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Open an image and save it as a PSD. My example peacock image is an RGB file,
but this technique is equally effective
on CMYK files. Lab sharpening
is also preferable over USM for
low-key images (where most of
the detail is in the shadows) and
also for images with lots of green
detail such as grass, leaves, and
other flora.
Click
the Create new document
from current state icon at
the bottom of the History
palette. With the new document
active, select Image
> Mode > Lab Color. Now
you’ll have two identical
versions of the image; one in
RGB Color mode and one in
Lab Color mode.
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In the RGB image, duplicate
the Background layer by
dragging its thumbnail over
the Create a New Layer icon
at the bottom of the Layers
palette. Name the new layer
RGB USM. With the new
layer active, select Filter >
Sharpen > Unsharp Mask
and use an Amount of 200%
and a Radius of 10 pixels.
This image is deliberately
oversharpened to better
illustrate what’s happening.
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With the Lab image active,
open the Channels palette. The
channels in a Lab image are very
different from the channels in
an RGB or CMYK image. The
image detail (also called the
luminance) is separated out into
a channel called Lightness, and
its color is separated out into
two channels called a and b.
The a channel holds the green
and red information, and the
b channel holds the blue and
yellow information.
Click the
Lightness channel and notice the
display turn to grayscale. This is
because all you’re seeing is the
luminance information, independent
of any color information.
Click on the visibility icon for
the composite (topmost) channel.
This allows you to see the
image in color while still limiting
any changes you make to the
Lightness channel. Apply an USM
on the Lightness channel using
the same settings from the previous step.
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You can’t have more than one color
mode in a document, so you’ll need to
convert the Lab file back to RGB Color
mode by selecting Image > Mode >
RGB Color. With the Layers palette visible,
Shift-drag the layer thumbnail into
the original RGB image to maintain the
layer registration. Name the new layer
LAB USM.
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 Toggle the visibility of the LAB USM layer off
and on. In my example image, observing the blue designs on the feathers is useful.
On the RGB USM layer (shown at left, click to enlarge) they’ve been exaggerated
into an electric blue. When I turned off the
visibility of the RGB USM layer and toggled the
visibility of the LAB USM layer on (shown at right) there were no
electric blues, the designs were just a lot sharper. Try this on your own image.
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Doug Nelson is a freelance writer,
technical editor, and founder
of RetouchPRO.com, an online
community for photo retouchers.
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