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

The Sharpen filter (previously called Unsharp Mask) sharpens edges of the elements without increasing noise or blemish.

To prevent color distortion while sharpening, you can Decompose your image to HSV and work only on Value. Go to Colors → Components → Decompose…. Make sure the Decompose to Layers box is checked. Choose HSV and click OK. You will get a new gray-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 by using Compose. Go to Colors → Components → 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.

This filter is found in the main menu under Filters → Enhance → Sharpen (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 (blue), like an S if there is some blur (yellow).

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

Let us subtract the difference between blurredness intensity (green curve) and the intensity of the original image (black curve) to the intensity of the original image (black curve). We get the red curve, which is more abrupt: contrast and sharpness are increased. QED.

Unsharp mask was first used in silver photography. The photographer 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 both films are placed 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. The “Unsharp 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).