Unsharp Mask Filter USM

Figure 4-31 : Image before sharpening
Figure 4-33: Image after sharpening using "Unsharp Mask"

Of all the options Photoshop offers for sharpening digital images, Unsharp Mask (USM) is probably the most widely used. Most other sharpening tools are variations on the same theme. In order to sharpen your images effectively you need to understand the unsharp mask technique and its limitations. The filter can be found under Filter r Sharpen r Unsharp Mask, and the dialog (figure 4-32) has three controls: Amount, Radius, and Threshold, which function as follows.

Amount • This value determines how strongly edge contrast is increased. Start by trying values between 90 and 120 percent. Usable values can reach as much as 150 to 250 percent.

You can see the effects different values have in the image preview. The greater your image resolution, the higher the value you can effectively use.

Figure 4-32: : Unsharp Mask filter dialog.

Radius • This value determines the number of neighboring pixels that are affected by the sharpening process. Values of 0.5 to 1.0 are often sufficient to achieve effective results; too, the higher your image resolution, the larger the radius value you can effectively use. Too large a radius value can lead to hard-looking contrast effects and to the formation of image artifacts.

Figure 4-32: : Unsharp Mask filter dialog.

Threshold • This value defines how many pixels to consider as an "edge". Start at 0 and work up; larger values produce effective edge sharpening without accentuating image imperfections too much. For images with smooth gradients (e.g., skies), we usually use a value of 5-10.

Unfortunately, it is easy to sharpen too little or too much. Too much sharpening can cause significant image quality loss, especially in the form of black or white halo effects. We recommend that you start small and sharpen only a little until you get a feel for how sharpening affects your images.

The effects of sharpening depend on how an image is to be used (e.g., print or Web,), and it can take some time and a lot of experimenting to find the right settings. This is where the main limitation of the USM process comes in to play: all USM corrections affect the entire image. It is often desirable to sharpen different parts of an image to different degrees (e.g., the face and the hair in a portrait). It is difficult to avoid amplifying minor noise effects when using USM.

This is why there are a large number of specialized sharpening programs and plug-ins available for making sharpening easier and more effective. Most of these tools are based on edge masking techniques.

There is no single, universal solution to the problem of sharpening, so you will have to experiment to find out the best techniques and tools to use in each individual situation.

Figure 4-34: This image has been over-sharpened and displays obvious sharpening artifacts.
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