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I'm interested in getting into post-processing stuff, and I'm wondering about the Unsharp Mask operation. Specifically, there are 3 parameters involved:

  • amount
  • radius
  • threshold

What do these do? Should I just leave them on the default settings? If not, what are the criteria for modifying them in each direction?

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3 Answers 3

up vote 33 down vote accepted

Some background:
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 0.5. For an image that is to be printed at 300 ppi, it translates to a radius of 1.5.

When I sharpen images in the final step for publishing on my web site, I use these settings:

  • Amount: 50%
  • Radius: 0.5
  • Threshold: 2
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5  
+1 Excellent answer! The background information was informative. –  jrista Jul 25 '10 at 2:53

The unsharp mask will increase the perceived sharpness of an image by increasing the contrast of pixels next to each other. It does so by making darker pixels a bit darker, and brighter pixels a bit brighter.

  • The amount parameter will control how much darker or brighter the pixels will be made.
  • The threshold parameter will prevent the filter from having any effect unless the difference between pixels is large enough.
  • The radius parameter determines how large area around each pixel that will be analyzed when performing the calculations.

Different images will require different settings. I typically use (in Photoshop) radius as low as possible, amount at around 80%, and then experiment with the threshold so that the sharpening happens where it is intended.

If the amount is too high, the image will look "over sharpened" in an unnatural way. If radius is too high, there will be a halo effects in sharpened areas of the image and if the threshold is too low, it may make image noise become more apparent. If radius or amount is too low or if threshold is too high, the result will be that there is no or little effect of the operation.

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Good, an answer that answers the question! –  labnut Mar 26 '11 at 7:15
    
+1, but reading this again, I think that the radius parameter does not correspond to the analysis phase (that is what the threshold is for) but for the effect application phase. –  ysap Jul 15 '11 at 15:35
    
+1 Thanks a lot! I think this answer is more practical in terms of what each settings mean and how changing each will it impact my image. –  Tejas Oct 9 '11 at 9:52

Bojidar Dimitrov has a great article about unsharp mask here. The article has example images and it also visualizes the parameters.

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1  
Can you summarize? –  mattdm Mar 26 '11 at 2:43
    
+1 for giving references. Too few people do this. –  labnut Mar 26 '11 at 7:16

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