If the focus on the camera is not set perfectly, or the camera is moving when the picture is taken, the result is a blurred image. If there is a lot of blurring, you probably won't be able to do much about it with any technique, but if there is only a moderate amount, you should be able to improve the image.

The most generally useful technique for sharpening a fuzzy image is called the Unsharp Mask. In spite of the rather confusing name, which derives from its origins as a technique used by film developers, its result is to make the image sharper, not “unsharp”. It is a plug-in, and you can access it as Filters->Enhance->Unsharp Mask in the image menu. There are two parameters, “Radius” and “Amount”. The default values often work pretty well, so you should try them first. Increasing either the radius or the amount increases the strength of the effect. Don't get carried away, though: if you make the unsharp mask too strong, it will amplify noise in the image and also give rise to visible artifacts where there are sharp edges.

Next to "Unsharp Mask" in the Filters menu is another filter called Sharpen, which does similar things. It is a little easier to use but not nearly as effective: our recommendation is that you ignore it and go straight to Unsharp Mask.

In some situations, you may be able to get useful results by selectively sharpening specific parts of an image using the Blur or Sharpen tool from the Toolbox, in "Sharpen" mode. This allows you to increase the sharpness in areas by painting over them with any paintbrush. You should be restrained about this, though, or the results will not look very natural: sharpening increases the apparent sharpness of edges in the image, but also amplifies noise.

When you take pictures in low-light conditions or with a very fast exposure time, the camera does not get enough data to make good estimates of the true color at each pixel, and consequently the resulting image looks grainy. You can “smooth out” the graininess by blurring the image, but then you will also lose sharpness. There are a couple of approaches that may give better results. Probably the best, if the graininess is not too bad, is to use the filter called Selective Blur, setting the blurring radius to 1 or 2 pixels. The other approach is to use the Despeckle filter. This has a nice preview, so you can play with the settings and try to find some that give good results. When graininess is really bad, though, it is often very difficult to fix by anything except heroic measures (i.e., retouching with paint tools).

Every so often you have the opposite problem: an image is too crisp. The solution is to blur it a bit: fortunately blurring an image is much easier than sharpening it. Since you probably don't want to blur it very much, the simplest method is to use the “Blur” plug-in, accessed via Filters->Blur->Blur from the image menu. This will soften the focus of the image a little bit. If you want more softening, just repeat until you get the result you desire.