Un-Sharp Mask (USM)

by Hugh Stockton (all rights reserved)

No matter how sharp the lens and perfect the exposure, all images are softened by the digitizing process. Edges fall across pixel boundaries so a given pixel is influenced by other parts of the image besides the edge. Using the USM filter provides selective contrast control across edges, which effectively "sharpens" the edges softened by digitizing. USM has three adjustments; Threshold, Radius, and Amount.

1. Threshold
   
   The Threshold adjustment determines how much contrast there must be between pixels to define an edge. Increasing Threshold can minimize the amount of noise added to a picture by the sharpening process.
   
   
2. Radius
   
   The Radius adjustment determines how wide will be the "valley" between the two sides of an edge.
   
   
3. Amount
   
   The Amount adjustment determines how deep will be the "valley" between the two sides of an edge.

The amount of each adjustment necessary to produce a given effect is determined by the size and resolution of the image. For a full sized D1x image, I start with:

• Threshold = 0,
• Radius = 1, and
• Amount = 100.

For an image that has been downsized to 800x520 and converted to 72 dpi, I start with:

• Threshold = 0,
• Radius = .3, and
• Amount = 50.

First convert the image to LAB (Image/Mode/Lab Color). If the Channels menu is not visible, open it (Window/Channels). Select the Lightness Channel. The image will change to gray scale.

Second, use the Navigator slider to zoom in about half way, and center the view window over an area of the image that has areas of high contrast or small structures. Open the Unsharp Mask tool. (Filter/Sharpen/Unsharp Mask).

Both the small window in the USM tool and the main window will show the image with the selected settings applied. Move the cursor over the small window and left click and hold. As long as you hold the click, the small window will display without the sharpening. Notice how the image changes with and without the sharpening. Note that the cursor has changed to a hand. You can drag the image to move the view window to areas of more detail.

Now click the Preview box off then on. The main image will revert to the non-sharpened state, then back to sharpened. Watch how the image changes with and without sharpening.

Watch particularly for halos around the high contrast edges. Halos indicate excessive sharpening. Increase Amount until halos develop, then decrease until they are minimal or acceptable. For very high resolution images, very large amounts, 300% or greater, may be necessary. In extreme cases, it may be necessary to increase Radius in order to accomplish the desired amount of sharpness. Increase in increments of 1/10 of a pixel (1 to 1.1, then to 1.2). I've never needed as much as 2 pixels.

Pay particular attention to noise patterns as you turn USM on, and off. If you see noise patterns, increase Threshold to remove them. Very small amounts of Threshold, 1 to 4, will typically be sufficient. Excessive Threshold will minimize the effectiveness of USM over the entire image.

When you are satisfied with the sharpness, click OK. Then change the image back to RGB (Image/Mode/RGB). Complete your workflow. If you started with a full sized, high resolution image you will need to downsize it for web use. Some sharpness can be lost in downsizing. If so, another application of USM can be effective.

Note that the smaller, lower resolution image will need much less Radius and Amount. As noted earlier, I start with:

• Radius = .3 and
• Amount = 50.
• The remainder of the process is the same.

Once complete, I suggest Save for Web at around 50% to 70%.