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<h3 class="title"><a id="id3314600"></a>6.4. Adjusting Sharpness</h3>
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<h4 class="title"><a id="id3314793"></a>Unblurring</h4>
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<p>
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.
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The most generally useful technique for sharpening a fuzzy image
is called the <a href="ch06s06s07.html" title="6.7. Unsharp Mask">Unsharp
Mask</a>. 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.
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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 <a href="ch06s03s16.html" title="3.16. Decompose">Decompose</a> and <a href="ch06s03s15.html" title="3.15. Compose">Compose</a> for more information.
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Next to "Unsharp Mask" in the Filters menu is another filter
called <a href="ch06s06s06.html" title="6.6. Sharpen">Sharpen</a>, 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.
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In some situations, you may be able to get useful results by
selectively sharpening specific parts of an image using the
<a href="ch03s03s09.html" title="3.9. Convolve (Blur/Sharpen)">Blur or Sharpen</a> 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.
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<h4 class="title"><a id="id3314403"></a>Reducing Graininess</h4>
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<p>
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 <a href="ch06s02s04.html" title="2.4. Selective Gaussian Blur">Selective Blur</a>, setting
the blurring radius to 1 or 2 pixels. The other approach is to
use the <a href="ch06s06s03.html" title="6.3. Despeckle">Despeckle</a>
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).
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<h4 class="title"><a id="id3314392"></a>Softening</h4>
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Every so often you have the opposite problem: an image is
<span class="emphasis"><em>too</em></span> 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 from the image menu. With the default
settings and a repeat count of 1, 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.
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