Sharpen soft or slightly blurry photos with amount, radius and threshold, and check the result at 100 percent before you save.
To sharpen an image, this tool applies an unsharp mask: it blurs a copy of the picture, subtracts the blur from the original to find the edges, and adds that difference back, scaled by the amount you choose. For one pixel the rule is new = original + (amount / 100) x (original - blurred). Take a pixel of brightness 120 whose blurred copy reads 100. The difference is 20. At an amount of 150 percent the new value is 120 + 1.5 x 20 = 150. The pixel on the dark side of the same edge gets darker by the same logic, so the edge looks crisper. In flat areas the original and the blur are equal, the difference is zero, and nothing changes. The blur is a Gaussian approximation (three box-blur passes) whose spread is the radius, in pixels. A small radius sharpens fine detail such as eyelashes and text. A large radius lifts broad outlines and gives a punchier look. The amount runs from 0 to 500 percent, the radius from 0.5 to 10 pixels.
The threshold is the smallest difference, in levels from 0 to 255, that gets sharpened. With a threshold of 25, the pixel in the example above stays at 120, because its difference of 20 is below 25. Raising the threshold leaves smooth areas such as skin and sky alone and sharpens only clear edges, which keeps grain and JPEG blocks from being boosted. The default is 0, which sharpens everything. Brightness only is the default mode: the tool blurs one brightness value per pixel and moves red, green and blue by the same amount, so colours do not shift and coloured fringes do not appear. Each colour mode blurs and sharpens red, green and blue separately. It is stronger, and on strong edges it can add a coloured outline. Alpha, the transparency channel, is never changed, but the colour channels are blurred as they are, so soft transparent edges of a PNG can pick up a faint dark or light rim.
Sharpening raises contrast along edges. It does not recover detail that was never captured. A photo that is out of focus or shaken stays soft, only with harder-looking edges, and pushing the amount far above 200 percent makes bright halos along outlines and boosts noise. Sharpen last, after you resize, because shrinking a picture softens it and sharpening before it is partly wasted. The saved file is in the same format as the original. JPEG and WebP files are encoded again at quality 92, which is a second lossy save, and PNG stays lossless. Photo details such as location and camera model are not kept. A picture over 24 megapixels, or over 12 megapixels on a phone, is sharpened at a smaller size and the result says so. The live preview shows the middle of the first picture at 100 percent, so the radius you see is the radius that is applied.
The pictures are opened and sharpened by code running in your browser, in a background worker, so you can press Cancel and the page stays responsive. There is no upload step and no server that receives them. The page counts visits with Google Analytics, using rough groups such as 1 to 10 MB, never a file name or an image. Up to 50 images can be added at once, about 500 MB in total on a computer or 150 MB on a phone. They are sharpened one at a time to keep memory use low, and a very large batch may pause and ask you to download what is finished before it goes on.
Add the photo, keep Brightness only selected and start with amount 100, radius 1 and threshold 0. Look at the preview at 100 percent, raise the amount in steps of 25 until edges look crisp but no bright outline appears, then press Sharpen. A photo that is badly out of focus will not become sharp, only harder-edged.
A gentle start for a 12 megapixel photo on a screen is amount 100, radius 1, threshold 0. For a 24 megapixel photo try a radius of 1.5 to 2. For small web images use amount 60 to 100 and radius 0.5 to 0.8. Fine detail such as text takes a small radius, broad outlines a larger one. Judge by the 100 percent preview, not by numbers alone.
Halos are bright or dark outlines that appear when the amount is too high for the radius, usually above 200 percent. Noise appears because sharpening lifts grain along with detail. Lower the amount, shrink the radius, or raise the threshold to 5 to 15 so only clear edges are sharpened and smooth areas stay clean.
It sets the smallest difference, from 0 to 255 levels, between a pixel and its blurred copy that is sharpened. At threshold 10, any pixel whose difference from its blur is below 10 levels is left as it was. This protects skin, sky and other smooth areas, while strong edges are still sharpened.
The format stays the same: JPEG stays JPEG, PNG stays PNG, WebP stays WebP. JPEG and WebP are saved again at quality 92, and sharpened detail is harder to compress, so the file is often somewhat larger than the original. The result row shows the real size before you download.
No. The images are sharpened by code running in your browser, in a background worker, and nothing is uploaded. The page counts visits with Google Analytics using rough size groups such as 1 to 10 MB, never a file name or an image. Your amount, radius and threshold are remembered in this browser.