Using Unsharp Mask

The Unsharp Mask (USM) filter has been used to sharpen images for many years and probably will continue to be used for some time to come because of its inclusion in actions, available freely and commercially, used on a daily basis.

On the face of it, the filter looks deceptively easy to use, but in practice it trips up many a traveler. Briefly, the Amount slider controls the strength of the halo, the Radius the width, and the Threshold the intensity. The key to understanding its interface lies not in just the effect each control has on the image but also the order in which they are used.

When you enter the dialog box, it's usually a good idea to find the appropriate width of the halo first by setting Amount to 400-500% and Threshold to 0%. Having found the optimum radius setting, decrease the Amount and then increase the Threshold incrementally to decrease the intensity of the halo. You would think that increasing an intensity value would increase the intensity, but in the case of Threshold, the opposite is true.

Rather than apply sharpening to the original layer, apply it to a copy. This way, you can increase your options to modify the effect. For example, you can reduce the opacity and lessen the effect, change the layer blend mode to Luminosity and ensure that the color is unaffected, or reduce the halos by using the Blend If controls found in the Layer Style dialog box (Layer ^ Layer Style ^ Blending Options). To do so, move the white slider for This Layer inward to limit a white halo or the black slider to limit the black halo. You can Alt-click (Windows), Opt-click (Mac OS) to split the sliders; when you slide them apart, the blend transitions less harshly (see Chapter 6 for more information on blending layers).

The filter works on only one layer at a time; if your content is spread over several layers, stamp the layers into a new layer. The quickest way to do that is by pressing Ctrl+Alt+Shift+E (Windows), S+Opt+Shift+E (Mac OS).

To use the Unsharp Mask filter, take the following steps:

1. Make sure that the layer you want to sharpen is active and the display magnification is set to 100% (View ^ Actual Pixels) if outputting to a monitor, or 50% or 25% if outputting to an offset printer. You do this because some of the illusion of sharpness can be reduced when an image is halftoned; viewing at a lower magnification can help you judge the end effect (do not view at fractional magnifications, such as 66.67%, 33.33%, and so forth, because the anti-aliasing applied by Photoshop at fractional magnifications can cloud your judgment).

2. Select Filter ^ Sharpen ^ Unsharp Mask.

3. Drag the Amount slider to 500%, the Radius slider to 0%, and Threshold to 0%.

4. Drag the Radius slider to the right or click in the text field and then use the Up and Down Arrow keys to increase/decrease the radius incrementally. There is no formula that works for all images; use your judgment to find the best value.

5. Reduce the value of the Amount slider (you may have to juggle Radius and Amount to find the best setting).

6. Increase the Threshold value incrementally if the image contains large areas of color that you do not want to sharpen, such as skin or sky. Avoid setting values too high, because doing so will result in reduced sharpening.

7. Click the OK button to exit.

While the dialog box is open, you can click in the proxy window to see a before and after view. You also have the luxury of judging the sharpness by referring to the document window, which provides a live preview and can be toggled off/on with the Preview button. If your resources are low or you are sharpening particularly large files, you might wish to deselect the live Preview to speed the process and use the option only after you have established the settings.

Images containing fine, fractal detail, such as leaves and grass, require a lower Radius setting. This is because the edges tend to be very narrow in such images. The reverse is true of images containing broad washes of color and fewer or wider edges, such as cars and faces, which might also benefit from a higher Threshold setting to exclude noise, grain, and skin blemishes from being sharpened.

Figure 11-22 shows the various phases of the sharpening process.

Figure 11-22: The Unsharp Mask filter dialog showing the various phases of the sharpening process. The color chips at the top indicate the halo width and intensity. A—All values set to their lowest. B—The Amount value set to 500%, in order to better judge the next step. C—The Radius value increased incrementally. D—The Threshold value increased to show its effect: the skin has been softened but so has the hair.

Figure 11-22: The Unsharp Mask filter dialog showing the various phases of the sharpening process. The color chips at the top indicate the halo width and intensity. A—All values set to their lowest. B—The Amount value set to 500%, in order to better judge the next step. C—The Radius value increased incrementally. D—The Threshold value increased to show its effect: the skin has been softened but so has the hair.

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