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. |