What sharpening actually does
Almost every result for “sharpen image” is now an AI tool. That is a different thing. Classic sharpening, the unsharp mask in Photoshop, GIMP and Lightroom, doesn’t invent anything: it compares the picture with a slightly blurred copy of itself, finds where the brightness changes, and makes that change steeper. Edges and textures look crisper; flat areas stay as they were. AI sharpeners instead redraw the picture with detail that looks plausible, which can quietly change a face, the letters of a number plate or the text in a document.
We measured how much a sharpener can bring back
We took four real photos, blurred them by 1.5 pixels to imitate slight misfocus, and ran them through this page at 100% and a 1.5 px radius. Then we compared both the soft and the sharpened version with the original, sharp photo.
| Photo | Edge contrast: soft → sharpened (original) | Closer to the original |
|---|---|---|
| Landscape, 1200×900 | 5.0 → 6.8 (25.9) | 5% |
| Harbour, phone | 2.4 → 3.3 (12.5) | 4% |
| Boardwalk, phone HDR | 6.2 → 8.3 (21.3) | 11% |
| Autumn leaf close-up | 2.9 → 3.7 (4.8) | 6% |
Edge contrast rose by 27–41%, which is what you see as “crisper”. But the photos got only 4–11% closer to the real thing, because fine detail that the blur removed is simply not in the file any more. That is the honest limit of sharpening: it is excellent for a photo that is a little soft, and it cannot rescue one that is really out of focus. Use the before and after crops, which show the busiest part of your picture at actual size, to judge the effect; a small thumbnail hides it completely.
Why only brightness is sharpened
Sharpening red, green and blue separately sharpens each colour by a different amount where two colours meet, and the edge gets a coloured fringe. On our test pattern of red and green stripes, per-channel sharpening at 200% shifted the hue of the colours along the edges by up to 10.2 degrees. This page sharpens the brightness and adds the same change to all three channels, so the hue shift was 0.3 degrees, which no one can see. Edges get crisper without changing colour.
The three settings
- Amount is how much steeper each edge gets. 100% suits most photos; go higher for prints viewed from a distance.
- Radius is how wide the edges are that get treated: 0.5–1 px for fine detail such as hair and text, 2–3 px for big prints or a picture that was enlarged. Too large a radius makes outlines thick.
- Ignore fine noise is the threshold: brightness differences smaller than this many levels are left alone, so skin, sky and JPEG noise don’t turn gritty. It fades in rather than switching on, so there is no visible cut-off.
If a bright outline appears around dark objects, that is a halo: lower the amount or the radius. The page warns you when more than 0.2% of the pixels are being pushed to pure black or white.
Sharpen last, and expect a slightly larger JPG
Resize, crop and brighten first, and sharpen at the final size; making a picture smaller softens it again. Crisp edges also take more data to store: in our tests the same photo saved at the same JPG quality came out 10–13% larger after sharpening at 100%.
Related tools
For a dark photo, brighten it first. To make the picture smaller before sharpening, use Resize image; to get it under an upload limit afterwards, Compress image to an exact size; and to raise the DPI number a print shop asks for, Change image DPI.