Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

Figure 9-2. The Unsharp Mask filter

Figure 9-3. Varying the USM Amount setting

Working the Controls," later in this chapter).

Radius

Radius is the first thing to consider when you're setting up sharpening; it sets the width of the halo that the filter creates around edges (see Image Detail and Sharpening Radius," later in this chapter).

Note that a Radius value of 1.0 does not result in a single-pixel radius. In fact, the halo is often between four and six pixels wide for the whole light and dark cycletwo or three pixels on each side of the tonal shift. However, it varies in width depending on the content of the image.

Threshold

Unsharp Mask only evaluates contrast differences: it doesn't know whether those differences represent real edges you want to sharpen, or areas of texture (or, even worse, scanner noise) that you don't want to sharpen. The Threshold control lets you specify how far apart two pixels' tonal values have to be (on a scale of 0 to 255) before the filter affects them (see Figure 9-5). For example, if Threshold is set to 3, and two adjacent pixels have values of 122 and 124 (a difference of two), they're unaffected.

You can use Threshold to make the filter ignore the relatively slight differences between pixels in smooth, low-contrast areas while still creating a halo around details that have high-contrast edges. And, to some extent at least, you can use it to avoid exaggerating noisy pixels in shadow areas.

Low Threshold values (0 to 4) result in a sharper-looking image overall (because fewer areas are excluded). High values (above 10) result in less sharpening, but often produce unnatural-looking transitions between the sharpened and unsharpened areas. We typically start out with a zero Threshold value, and then increase it only if necessary.

Tip: The Preview Checkbox

The Preview checkbox applies the Unsharp Mask filter to the entire image or selection on the fly, but we often keep this turned off. Even on fast machines, the preview can take a long time on large files, and every time you change the filter settings, Photoshop has to recalculate and redraw the entire screen. Of course, the larger the image, the longer it takes. We use Preview for quick before-and-afters, and when we want to check the final settings on the whole of the visible image.

Tip: Select a Critical Area to Preview

When you're working interactively with the Unsharp Mask settings, you may want to strike a happy medium between relying on the tiny proxy and previewing the whole image. You can do this with a few additional steps:

This is a tip that takes longer to explain than it does to do!

Filters and Effects" in Chapter 12, Essential Image Techniques). This is one of the best uses of Fade we've come across.

Tip: Fade Luminosity Instead of Convert to Lab

Some people like to sharpen images by converting them to Lab and sharpening only the Lightness channel. You can get an almost identical result by sharpening the RGB file, then using the Fade command from the Edit menu, with the Mode set to Luminosity. It's faster, and kinder and gentler on the image.

Everything's Relative

One of the most important concepts to understand about sharpening is that the three values you can set in the Unsharp Mask dialog box are all interrelated. For instance, as you increase the Radius setting, you generally need to decrease Amount to keep the apparent sharpness constant. Similarly, at higher Radius settings, you can use much higher Threshold values; this smooths out unwanted sharpening of fine texture, while still applying a good deal of sharpness to well-defined edges.