What do Amount, Radius, and Threshold do in Unsharp Mask?
Asked 7/24/2010
10 views
2 answers
0
I’m learning image post-processing and want to understand the Unsharp Mask sharpening filter. It has three controls: amount, radius, and threshold.
What does each one do in practice? Are the default settings usually fine, or should they be adjusted depending on the image? If they should be changed, what does increasing or decreasing each setting affect?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
16y ago
2 Answers
50
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 0.4. For an image that is to be printed at 300 ppi, it translates to a radius of 1.2.
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
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
16y ago
0
Generated from our catalog & community — verify before relying on it.
Unsharp Mask boosts perceived sharpness by increasing local contrast along edges: dark pixels are made a bit darker and bright pixels a bit brighter.
- amount: how strong the sharpening effect is. Higher values increase edge contrast more, but too much can look harsh.
- radius: how wide an area around each pixel is used to detect and enhance edges. A small radius affects fine detail; a larger radius creates broader edge contrast.
- threshold: how different neighboring pixels must be before sharpening is applied. Higher threshold protects smooth areas from being sharpened, reducing the chance of sharpening noise or subtle texture.
Defaults are not always best. Different images need different settings.
A practical approach is:
- start with a low radius
- set amount to a moderate level
- then adjust threshold so sharpening mainly affects real edges rather than flat areas
If sharpening looks exaggerated, lower the amount and/or radius, or raise the threshold. If the image still looks soft, increase amount carefully or slightly increase radius depending on whether you want to emphasize very fine detail or broader edges.
Recommended products
UniqueBot
AI16y ago
Your Answer
Related Questions
How do GIMP 2.10 unsharp mask "Standard Deviation" and "Scale" relate to radius and amount?
What does sub-pixel radius sharpening mean, and how is it different from regular sharpening?
How should I resize and sharpen photos for the sharpest prints?
What’s the difference between image contrast and image sharpening?
Why does Photoshop sharpening look weaker after I click OK?