Out-of-focus photographs and most digitized images often need a sharpness correction. This is due to the digitizing process that must chop a color continuum up in points with slightly different colors: elements thinner than sampling frequency will be averaged into an uniform color. So sharp borders are rendered a little blurred. The same phenomenon appears when printing color dots on paper.

The Unsharp Mask filter (what an odd name!) sharpens edges of the elements without increasing noise or blemish. It is the king of the sharpen filters.

Some scanners apply a sharpen filter while scanning. It's worth disabling it so that you keep control on your image.

To prevent color distortion while sharpening, Decompose your image to HSV and work only on Value. Then Compose the image to HSV. Go to Image/Mode and click on Decompose. Make sure the Decompose to Layers box is checked. Choose HSV and click OK. You will get a new grey-level image with three layers, one for Hue, one for Saturation, and one for Value. (Close the original image so you won't get confused). Select the Value layer and apply your sharpening to it. When you are done, with that same layer selected, reverse the process. Go to Image/Mode and click on Compose. Again choose HSV and click OK. You will get back your original image except that it will have been sharpened in the Value component.

You can find this filter through Filters → Enhance → Unsharp Mask.

Using an unsharp mask to sharpen an image can seem rather weird. Here is the explanation:

Think of an image with a contrast in some place. The intensity curve of the pixels on a line going through this contrast will show an abrupt increase of intensity: like a stair if contrast is perfectly sharp, like an S if there is some blur.

Now, we have an image with some blur we want to sharpen (black curve). We apply some more blur: the intensity variation will be more gradual (green curve).

Let us subtract the blurredness intensity from the intensity of the image. We get the red curve, which is more abrupt : contrast and sharpness are increased. QED.

Unsharp mask has first been used in silver photography. The photograph first creates a copy of the original negative by contact, on a film, placing a thin glass plate between both; that will produce a blurred copy because of light diffusion. Then he places both films, exactly corresponding, in a photo enlarger, to reproduce them on paper. The dark areas of the positive blurred film, opposed to the clear areas of the original negative will prevent light to go through and so will be subtracted from the light going through the original film.

Voilà. TheUnsharp Mask plug-in does the same for you.

At the beginning of the curve, you can see a dip. If blurring is important, this dip is very deep; the result of the subtraction can be negative, and a complementary color stripe will appear along the contrast, or a black halo around a star on the light background of a nebula (black eye effect).