When we're trying to determine the right sharpening settings for an image, we start by setting the Radius. We usually start out with exaggerated Amount and Threshold settings (400 percent and 0), then we experiment with the Radius value. The exaggerated Amount and Threshold make it easy to see what's happening as we adjust the Radius (see Figure 9-6).
Figure 9-6. Radius versus Amount versus Threshold
As you increase the Radius, the apparent sharpness also increasesoften to an undesirable extent. This is where the aesthetic considerations come in. Some people like more sharpening than others. We find oversharpened images more disturbing than slightly soft ones, but that's a matter of taste. It's up to you to decide how much sharpening you want.
However much sharpening you decide to apply, you'll find that as you increase the Radius setting, you need to decrease the Amount to keep the apparent sharpness constant. You can work these controls in opposition to achieve a wide range of sharpening effects.
Threshold is the third part of the equation. You can think of it as a selective smoothing function. At small (less than 1 pixel) Radius settings, a Threshold value as low as 15 or so will probably wipe out most of the sharpening effect. At higher Radius settings, you can use much higher Threshold values to smooth out unwanted sharpening of fine texture, while still applying a good deal of sharpness to well-defined edges.
There are dangers lurking here, though. As you use higher Amount and Threshold settings, you run an increased risk of driving pixels to solid black or solid white. The solid black ones aren't usually too much of a problem, but the blown-out white ones can appear as noticeable artifacts, especially when they're large due to higher Radius settings.
With higher Threshold settings, you get dramatic unnatural sharpening of high-contrast edges, while leaving smaller details soft. This makes the image look quite disturbingit's hard for the eye to reconcile the sharp edges and the soft detail, so the image looks like there's something wrong with the focus.
In short, the three parameters provided by Unsharp Mask give you a lot of control over the sharpening effect, but it takes a while to get your head around the way they interact.