Sharpening With Unsharp Mask

Unsharp Mask sharpens an image by finding an edge (line within an image) and making the dark side of the edge darker and the light side of the edge lighter. Unsharp Mask has three adjustable parameters: %, pixels, and threshold.
% is the percentage of lightening and darkening (typically 100 - 300%).
Pixels is the number of pixels wide that USM looks for an edge (typically 0.5 - 1.5 for a digital camera image or 1-3 for a scanned slide).
Adjacent pixels must differ by more than the threshold to be sharpened by USM. Typical threshold values are 1-2 for a rock wall or 15-20 for a smooth skin tone.
Oversharpening causes halos around the sharpened edges.