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“ Follow along at home: Start up Photoshop. Recreate your own simple gray block document to match the example (above). The lefthand block should have an RGB value of 86, 86, 86 and the righthand block a value of 43, 43, 43. If you use the Paint Bucket tool to create the blocks, make sure that anti-aliasing is off. You want solid blocks of a single color value. Sharpening makes the edge of the lighter block a lighter value of gray, and the edge of the darker block a darker value of gray. Select Unsharp Mask from the Filters menu and set starting values of 100 for Amount, 2 for Radius, and 0 for Threshold. Amount: determines the aggressiveness of the “sharpening” action. With your simple two-gray image, try amounts of 100, 200, and 400 (make sure the Preview box is checked in the Unsharp Mask dialog so that you see the changes as you make them; you should also be viewing at Actual Pixels size). What you should see is that as the amount is increased, the colors of the new edges get more exaggerated. In other words, the light line that gets added on one side of the boundary gets lighter with each increase, the dark line on the other gets darker (though that’s often more difficult to see).
Radius: determines how wide an area at the transition is affected. Try increasing the Radius to 4 and 8, and you’ll see that area that is modified at the transition widens. Note, too, that the further away from the actual transition point you get, the less the Amount is applied.
Threshold: determines how much difference there must be between two adjacent pixels before any change is made. In our simple example, you’ll have to enter very high numbers before you see how this works (try 25, 50, and 100). Note that threshold and radius interact a bit. With a Threshold of 100 and a Radius of 1 or less, almost nothing changes, but if you increase the Radius, you’ll start to see the effect again