The unsharp mask is an old technique that has been used in darkrooms long before computers were capable of processing images.
The original process consists of two exposures; first you create an unsharp mask by making a contact copy on low-contrast positive film, but with a distance between the original and film (and sometimes a diffusing plastic sheet) so that it becomes blurred. When you put the blurred positive and the negative together and make an exposure on paper, the positive cancels out some of the light from the negative. As the positive is blurred, it lets through more of the small details, which makes the image appear sharper.
Two of the parameters directly correspond to this old method;
- The radius is how much the positive is blurred.
- The amount is how much the positive is exposed.
The threshold has been added to enable you to exclude low-contrast edges from the sharpening.
A good baseline for the radius is around
0.1 mm. For an image that is to be viewed on a screen, which is about
100 ppi, it translates to a radius of
0.4. For an image that is to be printed at
300 ppi, it translates to a radius of
When I sharpen images in the final step for publishing on my web site, I use these settings after rounding:
- Amount: 50%
- Radius: 0.5
- Threshold: 2