Sharpening an Image with Unsharp Mask

Unsharp mask is a misleading term as it is actually a very good sharpening filter. The following was taken directly from Photoshop's documentation:

"Unsharp masking, or USM, is a traditional film compositing technique used to sharpen edges in an image. The Unsharp Mask filter corrects blurring introduced during photographing, scanning, resampling, or printing. It is useful for images intended for both print and online viewing.

Unsharp Mask locates pixels that differ from surrounding pixels by the threshold (how much different in contrast) you specify and increases the pixels' contrast by the amount you specify (amount input box). In addition, you specify the radius of the region to which each pixel is compared (how large is the region of comparison). The effects of the Unsharp Mask filter are far more pronounced on-screen than in high-resolution output. If your final destination is print, experiment to determine what settings work best for your image"

Careful adjustment of the tool's three parameters  'Amount' - 'Radius' and 'Threshold' will give the illusion of an overall sharpening of the image even though you are only adding contrast to pixels in adjoining areas of the image that already have markedly differing contrast. These areas are what Adobe refers to as 'edges'. A good place to start your experimentations would be 50 to 80 % for 'Amount' and between .4 to 1.5 as a 'Radius' and leave 'Threshold' between 1 and 3. High resolution images generally will require larger 'radius' numbers between 1.5 to 3. In all cases raising the 'amount' and/or 'radius' input will give more edge sharpening affect. Raising the 'threshold' will diminish the edge sharpening affect.

Steve Hoffmann's Nature and Landscape Photography