4. Sharpening
There are many and varied things in the Filters menu:
These filters apply effects that vary from the mundane and useful to the utterly bizarre. I recommend playing with them to learn more. Here, I'll just talk about a couple of filters that are useful for sharpening images-- something you'll probably want to do a lot, since digital camera images often come out looking a little soft, either because of focus or camera-shake problems, or simply because of the nature of the color digicam sensor. Anyway, the relevant filters are Sharpen and Unsharp Mask under Filter->Enhance.
Sharpen is pretty simple:
You've got a single slider controlling the amount of sharpening, and a little preview window (too little, if you ask me). You can change the part of the image that the preview previews by using the scroll bars. Clicking OK makes it go.
The key here is subtlety. Digital sharpening can help a lot, but there's only so much that it can really do; you can't bring back information that is completely gone. And beginners often overdo it and make an image that is sort of artificial-looking and grainy. Keep in mind also that many digicams do something like this internally already.
A more versatile, often more natural-looking, and, in some cases, more powerful sharpening filter is the oddly named Unsharp Mask:
This has three controls. The middle one, Amount, controls the amount of sharpening, pretty much as in the Sharpen filter, though the scaling is a little different; I find that I typically need less here.
The Radius control is new. Sharpening filters work by heightening the contrast near an edge, and Radius controls the width of those bands of heightened contrast. If the image is really blurry, or you're trying to bring out faint details in a washed-out image, you might want to increase this to several pixels; astronomers trying to boost the terrain contrast in space-probe images will often crank it up to dozens or hundreds of pixels. Under most digicam circumstances, I find that about 2 pixels is enough, if that.
The Threshold control is useful for preventing an unfortunate side effect of a simple sharpening filter, which is that it can exaggerate the noise in an image and make it look grainy. Brightness differences that are below the threshold will not be sharpened. If you adjust this right, you can trap the noise below the threshold and leave only the genuine image edges. Usually it needs to be pretty low for the filter to be useful.
Unfortunately, this dialog doesn't have a preview, so you'll probably end up fumbling around and undoing repeatedly. Remember, Control-Z is your friend. (With these image filters I find myself repeating Control-Z (undo) and Control-Y (redo) in rapid succession, to make sure I know which version I like better. It's like visiting the optometrist: better 1 or better 2?)
(Trivia moment: Why is it called Unsharp Mask? The reason is deep in the history of traditional photography. Believe it or not, the unsharp mask was invented not as a digital filter but as a physical darkroom technique. Basically, you're making the picture sharper by masking it slightly with a blurred version, to mask out some of the unsharpness. That's really how it works, in physical or digital form.)
Next: Controlling image size