Unsharp Mask

Although it sounds backwards, “Unsharp Mask” in most cases will enhance clarity and make images sharper and clearer. While this filter may appear to have an affect the entire image, Unsharp Mask is only applied to the edges of an image. It accomplishes the “sharpening effect” by adjusting the acutance, which refers to the clarity or transition at the edge of an image. Unsharp Mask is a great way to increase clarity of detail in photographs.

This filter should not be applied to every image, but only on images that need a crisp sharp look. If enhanced detail is not needed in an image, using this filter could result in unwanted noise or added graininess to the look to the image.

Images that are great candidates for this filter would be images that contain a large amount of detail. While there is a large range available in the setting, small amounts of these settings accomplish the sharpening need. Applying this filter too much can over emphasize certain textures and unwanted detail, as well as, in some instances create a halo around images. All of which would be unwanted results.

One thing to always remember about this filter is it is not a solution for a photo that is not in focus. While it may look better with the application of the filter, it won’t make a out of focus photo sharp. Another thing to keep in mind is that it should be applied to images last, after all other editing has been done. This will ensure maximum clarity.

Some suggested setting for unsharpen mask.

Subject              Amount         Radius          Threshold
Soft subjects       150                 1                     10
Portraits             75                   2                        3
Moderate sharpening 225       0.5                      0
Maximum sharpening 65         4                       3
All-purpose sharpening 85      1                        4


Source:
For starting values of the USM filter for different uses, try the ones suggested by PhotographyJam:
http://photocritic.org/unsharp-mask-sharpening-photos-photoshop/



Resources:
http://www.earthboundlight.com/phototips/unsharp-mask.html
http://www.cambridgeincolour.com/tutorials/sharpness.htm
http://www.cambridgeincolour.com/tutorials/unsharp-mask.htm
http://dictionary.reference.com/browse/acutance
http://www.naturescapes.net/122004/tg1204.htm