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.
Sometimes using Unsharp Mask can cause color distortion where there
are strong contrasts in an image. When this happens, you can often
get better results by decomposing the image into separate
Hue-Saturation-Value (HSV) layers, and running Unsharp Mask on the
Value layer only, then recomposing. This works because the human eye
has much finer resolution for brightness than for color. See the
sections on Decompose and
Compose for more information.
Next to "Unsharp Mask" in the Filters menu is another filter called
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
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
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
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.