Unsharp masking (often abbreviated as USM) may sound like the last thing you'd want to do if you're trying to make an image appear sharper, but the term actually makes some sense; it has its origins in a traditional photographic technique for enhancing sharpness.
The things we see as edges are areas of high contrast between adjacent pixels. The higher the contrast, the sharper the edges appear. So to increase sharpness, you need to increase the contrast along the edges.
In the traditional process, the photographic negative is sandwiched in the enlarger along with a slightly out-of-focus duplicate negativean unsharp maskand the exposure time for printing is approximately doubled. Because the unsharp mask is slightly out of focus and the exposure time has been increased, the light side of the edges prints lighter and the dark side of the edges prints darker, creating a "halo" around objects in the image (see Figure 9-1).
Figure 9-1. Edge transitions and sharpening
As you'll see throughout this chapter, this halo effect is both the secret of good sharpening, and its Achilles' heeldepending on the size and intensity of the halo, and where it appears in the image. Photoshop lets you control the halo very precisely, but there's no single magic setting that works for all images; so you need to know not only how the controls work, but also what you're trying to achieve in the image.