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The Unsharp Mask filter has three settings: Amount, Radius, and Threshold. Understanding the impact of these three settings is important to creating the sharpening results you want.

The Amount setting controls how much sharpening (that is, increase in contrast between pixels) occurs. The Radius controls the width (in pixels) on both sides of a sharpened edge — and how it's affected by the sharpening. The Threshold determines where (that is, on which pixels in an image or selection) to apply the sharpening. This happens by setting a minimum (threshold) difference in grayscale value that must exist between two pixels before the Unsharp Mask can be applied to those pixels.

Here you should set the Amount to 100%, the Radius to 1 pixel, and the Threshold to

3 pixels. These values noticeably sharpen the wing pattern, but leave the spaces between the pattern lines smooth.

Note: Keep in mind that a little sharpening goes a long way! It's easy to oversharpen an image. Because sharpening increases contrast tti

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along high contrast pixels, it is easy to push the difference in luminance too far, which creates halos. For instance, if we apply an amount of 200% rather than 100% to the dragonfly wings, halos develop along the edges of the wing filaments. The accompanying figures show the results of various degrees of sharpening, going from no sharpening to 100% sharpening to 200% sharpening

Note the halos in the 200-percent-sharp-ened image, a sure sign of oversharpening. The amount of sharpening you should apply depends upon the resolution and content of the image as well as on the effect you want to create. In general, however, it's a good idea to zoom in on your image so you can view the actual impact of the sharpening before you commit to a specific set of sharpening values.

But because high-contrast edges are softened more than smooth areas, if you apply the same sharpening to smooth areas that you do to edges, you may over-sharpen smooth areas, creating unwanted patterns. This is where the Threshold setting comes into play. When you set a minimum (threshold) value that must exist between two adjacent pixels before they can be affected by the sharpening process, you can restrict sharpening to just the higher-contrast edges.

Photoshop Confidential

The Need for Sharpening and Controlling It

The truth about digital image capture, whether by scanner or digital camera, is that digitizing softens images. Lots of averaging goes on in the digitizing process — but the smoothing process is not uniform. High-contrast edges are softened more than low-contrast areas. High-contrast edges become wider and smoother. And it is the sharpness of the high-contrast edges in an image that controls the focus in a digital image. Sharpening is basically a process of enhancing edge contrast: Increasing the contrast in grayscale values between adjacent pixels increases image sharpness.

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