I learned how to apply unsharp masking years ago and it can make and/or break an image. For those interested I'll try to explain how it works in Photoshop.
In order to be able to really use, and get the benifits of unsharp masking you have to start with a high quality, well exposed and close to full frame size image (not a cropped postage stamp sized portion of the image). The image should not appear grainy (low ISO setting), as depending on your settings you will highly amplify that grain.
An image is usually perceived as sharp if the edges in the image are well defined, reasonably crisp and have a good edge contrast. A soft image would have a smooth transition between edges and does not have a clear line of distinction.
In Photoshop, applying unsharp mask requires adjusting 3 different parameters. Too little USM may not add much to the sharpness of the image. Too much USM can add lot of grain to the image and makes the edges brighter than rest of the image making it look unreal.
The USM filter detects the edges in the image, and then creates halos around the edges. The Radius field is used to specify the size of the halo. The larger the specified radius, larger will be the halo produced at the edges. Keeping this in mind, it is good to use a small radius for an image with lot of details - such as feathers of a bird. A larger radius could be useful for images that do not have sharp edges and need a smooth transition. Images such as foggy mornings or sunsets do not have and do not require sharp edges or fine details and these are better off with slightly higher radius.
The threshold field indicates what part of the image should be considered an edge for applying USM. A threshold of 0 indicates every edge to be considered a candidate for USM. A higher threshold means that the edge contrast needs to be higher to consider that edge for applying the filter.
The Amount field indicats the amount of contrast to be applied on the halos created. A higher value amount means dark edge will be made darker and lighter edge is made to look lighter.
When shooting my marine images I start with an Amount of 150-200, a Radius of 1.2-2, and a Threshold of between 10 and 4, make sure the preview box is checked and you can observe the changes in real-time.
Hope this helps ... didn't mean to ramble ...
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