In the first installment of this series, we looked at the
theory of sharpening and how it works. Now let's take a look
at the Sharpen filters and the Sharpen tool.
Photoshop offers four Sharpen filters and a
single Sharpen tool. Each of the commands can be applied to
an entire image or to a selection. The tool is pictured here
as the active tool in the Toolbox (and is located below the
cursor).
We'll begin with a look at the Sharpen and
Sharpen More commands. Neither of these commands offers you
any level of control. They don't have dialog boxes; they
don't have previews. They each apply a given set of
sharpening parameters to the image, regardless of resolution
or tonality. They sharpen all pixels in an image. For those
reasons, they are seldom used. They can, however, do an
effective (and quick) job. Below is part of a picture that
is 72 pixels per inch (ppi), a low resolution image. It has
been divided into three sections. On the left is the
original, in the center the filter Sharpen has been applied,
and on the right the filter Sharpen More has been applied.
The results are viewed at 100%.
As you can see, the Sharpen command has done
a fairly adequate job, while the Sharpen More command has
over-emphasized the edges of the twigs and branches and has
created a texture in the sky. When the same general area of
the image is resampled to 250 ppi, the Sharpen command have
almost no noticeable effect, and even the Sharpen More
command seems to be a bit too little. (These filters can be
applied multiple times.)
The Sharpen Edges command, like Sharpen and
Sharpen More, has neither dialog box nor user-definable
settings. Unlike the others, however, it doesn't sharpen all
pixels. Sharpen Edges looks for area where a discernable
edge is present. The amount of sharpening applied, however,
is miniscule and the filter is rarely useful.
The Unsharp Mask command (sometimes referred
to as USM) gives you control over the sharpening process. It
is the most commonly-used sharpening tool because of its
three sliders and preview window. The dialog box is shown
below.
In addition to the Preview window shown, the
image itself is updated as you work. The three controllable
variables are:
AMOUNT: This determines the amount of contrast that will be
added where ever Unsharp Mask finds an edge. The value can
range from 1% to 500%. Typically, an Amount of 100% is a
good starting point for a low resolution image, while
150-200% is more appropriate for high resolution images.
RADIUS: Ranging from 0.1 pixel to 250
pixels, the Radius setting determines how wide an area will
be affected where Unsharp Mask finds an edge. (If you recall
the ÒhalosÓ discussed in Part 1 of this series, think of
Radius as the thickness of the halos.) For most images, 1 or
2 pixels is adequate to restore detail.
THRESHOLD: Threshold can range from 0 to
255, just as any other 8-bit value. It determines how
different much a pixel must vary from those neighboring to
be sharpened. Threshold is used to actually find the edges.
When 0 is chosen, Unsharp Mask will sharpen all pixels.
Threshold also helps prevent the filter from introducing
noise into areas of relatively uniform color. (Remember the
texture created in the background by the Sharpen More filter
in the second image?) Typically, a value between 2 and 20 is
entered.
Let's take a look at a comparison of two
Threshold settings. Both halves of the image below have
Amount set to 100% and Radius set to 1.5 pixels. On the
left, Threshold was set to 1; on the right, Threshold was
set to 6.
Notice the more subtle sharpening on the
right, and the lack of added texture in the background. On
the other hand, the smallest twigs and leaves at the ends of
the branches are not sharpened at all on the right.
The other option available to you for
sharpening is the Sharpen tool. The tool's options are shown
here. (The Options Bar has been shortened for convenience.)
The Sharpen tool uses brushes, and the brush
can be selected through the Brushes palette, just as with
any painting tool. The blending modes available are Normal,
Darken, Lighten, Hue, Saturation, Color, and Luminosity.
Pressure, measured in percent, determines the intensity of
the sharpening effect. When Use All Layers is checked, the
Sharpen tool will increase contrast along all edges found
below the brush. When unchecked, the tool is restricted to
the active layer. Normally found to the right of the Options
Bar, Brush Dynamics allows the Sharpen tool to fade either
the brush size or the pressure as you drag. It also offers a
Stylus option for use with drawing tablets.
In the next installment of this series,
we'll look at some specific sharpening techniques and ways
to avoid certain sharpening problems.