I have the Unsharp Mask formula burned into my head from design school years ago: Amount-100, Radius-1 px, Threshold-3. These amounts are, of course, just starting points but they work pretty well for most high resolution images. I still use Unsharp Mask from time to time but I've discovered a better way to sharpen. From the Photoshop Help file:
Sharpen Edges and Unsharp Mask: Finds the areas in the image where significant color changes occur and sharpen them. ... For professional color correction, use the Unsharp Mask filter to adjust the contrast of edge detail and produce a lighter and darker line on each side of the edge. This process emphasizes the edge and creates the illusion of a sharper image.
High Pass: Retains edge details in the specified radius where sharp color transitions occur and suppresses the rest of the image. (A radius of 0.1 pixel keeps only edge pixels.) The filter removes low-frequency detail from an image and has an effect opposite to that of the Gaussian Blur filter.
The most important thing to remember: "High Pass retains edge details ... and suppresses the rest of the image." This is what makes High Pass the better way to sharpen. Lower contrast areas like skin pores and background noise aren't sharpened the way they would be with Unsharp Mask. I've also found High Pass to be more effective than Unsharp Mask at sharpening slightly blurry details. The technique:
- Open your image in Photoshop.
- Duplicate the background layer.
- Apply Filter > Other > High Pass. A good range for low/medium resolution photos is .5 to 2 px and for high resolution photos try 3 to 6 px.
- Change the blending mode to Overlay (my favorite), Soft Light or Hard Light and reduce the Layer Opacity if you wish.
Note: If you get any weird color shifts (pretty rare), desaturate the layer (Cmd - Shift - U or Image > Adjustments > Desaturate).