How to use the image unsharp mask sharpener
- Choose an image. JPG, PNG, WebP, GIF, BMP and AVIF are accepted, up to 25 MB and 16 megapixels. SVG is refused.
- Set the radius in pixels, the amount in percent and the threshold in levels from 0 to 255. Tick Sharpen brightness only to avoid color fringes.
- Compare the before and after previews, which show a part of the picture at its true size.
- Choose Sharpen and download, as a PNG or a JPEG.
How an unsharp mask works
The name is old: darkroom workers made a blurred, low-contrast "unsharp" copy of a negative and combined it with the original. The digital version does the same. It blurs the picture with a Gaussian blur whose standard deviation is the radius, subtracts the blurred copy from the original to find the edges, and adds that difference back, multiplied by the amount. Where the picture is flat the difference is zero and nothing changes. Along an edge the dark side gets darker and the light side lighter, which the eye reads as extra crispness.
The threshold leaves small differences alone, so smooth skin, sky and noise are not boosted. In brightness-only mode the mask is computed on luminance and added equally to the red, green and blue values, which avoids colored halos. For an edge that steps from 50 to 200 with a radius of 1 and an amount of 100 percent, the last dark pixel goes from 50 to about 4.9 and the first light pixel from 200 to about 245.1.
Choosing settings and what to expect
A small radius, such as 0.5 to 1.5 pixels, sharpens fine detail. A larger radius, 2 to 5 pixels, adds local contrast to bigger shapes. An amount of 50 to 150 percent is a gentle start; high amounts create bright halos. Sharpening cannot add detail that the camera did not record, and it also boosts noise and compression artifacts, so use the threshold on grainy pictures.
To adjust light and contrast first, use the image brightness adjuster. To blur a face or a plate, use the blur image area tool. To shrink the file afterward, the image compressor is useful. Transparent areas keep their transparency; colors near transparent edges are weighted by opacity so they do not bleed.
Assumptions and limits
- The picture is changed in your browser at full size, in bands of rows, so large pictures do not use much memory.
- The radius is the standard deviation of the blur in pixels, cut off at about three times that value. Other programs may define radius differently.
- Threshold is the smallest difference, on a 0 to 255 scale, that gets sharpened.
- Metadata such as the camera and location is not copied to the saved file.
Frequently asked questions
Why does the preview show only part of the picture?
Sharpening is judged at the size you will view it. The preview shows a 1:1 crop from the center so you see real pixels. The saved file is sharpened over the whole picture with the same settings.
What causes bright halos along edges?
An amount or radius that is too high pushes the pixels beside an edge past the values of the edge itself. Lower the amount first, then the radius. The threshold does not remove halos.
Should I save as PNG or JPEG?
PNG keeps every pixel exactly but the file is larger. JPEG is smaller but compresses again, which can show as blocky noise near edges. Choose PNG if you will edit the picture further.
Does it work on screenshots or text?
It can, but sharp text and lines are already crisp, and a high amount creates halos around letters. Use a low amount and a small radius, or leave such images alone.