Unsharp masking
From Nikonians Wiki
Unsharp masking is an adaptive algorithm which places a slightly blurred version of the image behind the image, giving the impression of greater local contrast and hence greater sharpness. It is a technique traditionally used in a wet darkroom but now primarily applied in computer postprocessing.
Unsharp masking can be applied as capture sharpening and as final image sharpening.
In final image sharpening, it should be the last filter applied to the image, immediately prior to conversion to CMYK, if required, and after the image has been resized for its final purpose.
The formula for the optimal unsharp masking radius in final output sharpening is:
Viewing Distance inches x Resolution ppi x .0004
OR, as a rule of thumb, - resolution divided by 200 for normal viewing distances.
Unsharp masking can also be applied selectively as creative sharpening at any point in postprocessing.
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