Unsharp Mask
A sharpening method that subtracts a blurred copy of the image from the original to find and enhance edges.
Formula / rule
Sharpened = Original + Amount × (Original − Blurred)
Measured in
amount, radius, threshold
In depth
The name comes from a darkroom technique where a slightly out-of-focus positive was sandwiched with the negative. Amount sets the strength, radius the width of the affected edge, and threshold how much difference is needed before sharpening applies — raising threshold is what keeps smooth skin and sky from being sharpened along with genuine edges.
Examples in practice
An amount of 120, radius 0.8 and threshold 3 sharpens a portrait's eyes while leaving cheek texture untouched.