Sharpen a JPG

unsharp mask · radius, amount, threshold · luminance-only · batch · no upload

A real unsharp mask, not a fixed sharpening filter: it blurs a copy of the image, takes the difference — which is the detail — and adds a controlled amount of it back. That is what gives you a radius, an amount and a threshold, and what lets you sharpen the edges of a photo without also sharpening the sensor noise in the sky behind it. The mirror of Blur, and the last step after Resize.

Settings that work

• A photo for the web, after resizing — radius 0.5–1, amount 60–100%, threshold 2–4. Small radius because the detail left after downscaling is one pixel wide.
• A full-size photo from a camera — radius 1–2, amount 60–80%, threshold 3–6. Raise the threshold if the image is from a high ISO.
• A scan of a document or a screenshot of text — radius 1, amount 100–150%, threshold 0. There is no noise to protect and the edges are what you want.
• A hazy landscape — radius 5–10, amount 20–40%, threshold 0. At this radius it is not sharpening detail but adding local contrast, which is what "clarity" means in a photo editor.
• A JPG that has already been sharpened — don't. Sharpening twice produces halos that no third pass will remove.

How to tell you have gone too far

Look at a high-contrast edge — a roofline against the sky, dark text on white. A light band on one side and a dark band on the other is a halo, and it is the signature of too much amount or too large a radius. Then look at a flat area, sky or a wall: if it has come alive with speckle, the threshold is too low and you are sharpening noise. The status line reports what percentage of pixels the threshold allowed the tool to touch; on a noisy photo you generally want that number well under half.

What sharpening costs you

File size, mostly. A sharpened image has more high-frequency detail, which is precisely what JPEG compresses worst, so the same quality setting yields a larger file — sometimes 20–30% larger. That is the trade, and it is why sharpening belongs at the end of the pipeline: sharpen, then choose the quality, and if the result is too heavy use Compress to find the size you need rather than sharpening less.