Specifying the amount of sharpening

If Amount were the only Unsharp Mask option, no one would have any problems understanding this filter. If you want to sharpen an image ever so slightly, enter a low percentage value. Values between 25 and 50 percent are ideal for producing subtle effects. If you want to sharpen an image beyond the point of good taste, enter a value somewhere in the 300 to 500 percent range. And if you're looking for moderate sharpening, try out some value between 50 and 300 percent. Figure 10-8 shows the results of applying different Amount values while leaving the Radius and Threshold values at their default settings of 1.0 and 0, respectively.

If you're not sure how much you want to sharpen an image, try out a small value in the 25 to 50 percent range. Then reapply that setting repeatedly by pressing Ctrl+F. As you can see in Figure 10-9, repeatedly applying the filter at a low setting produces a nearly identical result to applying the filter once at a higher setting. For example, you can achieve the effect shown in the middle image in the figure by applying the Unsharp Mask filter three times at 50 percent or once at 250 percent. I created the top-row results in Figure 10-9 using a constant Radius value of 1.0. In the second row, I lowered the Radius progressively from 1.0 (left) to 0.8 (middle) to 0.6 (right).

The benefit of using small values is that they enable you to experiment with sharpening incrementally. As the figure demonstrates, you can add sharpening bit by bit to increase the focus of an image. You can't, however, reduce sharpening incrementally if you apply too high a value; you must press Ctrl+Z and start again.

Figure 10-8: The results of sharpening an image with the Unsharp Mask filter using eight different Amount values. The Radius and Threshold values used for all images were 1.0 and 0, respectively (the default settings).
Figure 10-9: Repeatedly applying the Unsharp Mask filter at 50 percent (top row) is nearly equivalent on a pixel-by-pixel basis to applying the filter once at higher settings (bottom row).

Just for fun, Color Plate 10-3 shows the results of applying the Unsharp Mask filter to each of the color channels in an RGB image independently. In each case, I maxed out the Amount value to 500 percent and set the Radius and Threshold to 4.0 and 0 respectively. The top row shows the results of applying the filter to a single channel; in the second row, I applied the filter to two of the three channels (leaving only one channel unfiltered). You can see how the filter creates a crisp halo of color around the chess pieces. Sharpening the red channel creates a red halo on the inside of the pieces and a blue-green halo on the outside; sharpening the red and green channels together creates a yellow halo on the inside and a bluish halo on the outside; and so on. Applying the filter to the red and green channels produced the most noticeable effects because these channels contain the lion's share of the image detail. The blue channel contained the least detail — as is typical — so sharpening this channel produced the least dramatic results.

Cross- If you're a little foggy on how to access individual color channels, read Chapter 4.

Reference^ Incidentally, you can achieve similar effects by sharpening the individual channels in a Lab or CMYK image.

As I mentioned in Chapter 4, Photoshop is ultimately a grayscale editor, so when you apply the Unsharp Mask command to a full-color image, Photoshop actually applies the command in a separate pass to each of the color channels. Therefore, the command always results in the color halos shown in Color Plate 10-3 — it's just that the halos get mixed together, minimizing the effect. To avoid any haloing whatsoever, convert the image to the Lab mode (Image ^ Mode ^ Lab Color) and apply Unsharp Mask to only the Lightness channel in the Channels palette. (Do not filter the a and b channels.) This sharpens the brightness values in the image and leaves the colors 100 percent untouched.

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