Sharpen an image with a proper unsharp mask
Photos often come out a little soft: the focus was slightly off, the phone smoothed away texture, or the picture was resized down for the web. The sharpen image tool restores crispness with an unsharp mask, the technique professional editors use, controlled by three sliders — amount, radius and threshold.
Sharpening is a fine-detail edit, so the preview has a 100% view that shows real pixels, plus a split line to compare with the original. By default only brightness is sharpened, which avoids coloured halos. Everything runs on your device at full resolution, with batch processing and a ZIP download for whole folders.
How to use the Sharpen Image
- Add one or more images by dropping them, clicking the upload area or pasting.
- Switch the preview to 100% and drag the picture to a detailed area such as eyes, text or leaves.
- Start with a preset, then fine-tune Amount (strength), Radius (size of the edges) and Threshold (noise protection).
- Drag the split line to compare. Stop when edges look crisp but before bright or dark outlines (halos) appear.
- Choose an output format and press Download sharpened image, or download a whole batch as a ZIP.
Sharpen Image features
Classic unsharp mask
Amount up to 500%, radius from 0.1 to 10 px and a threshold, with the exact formula shown as you adjust.
Brightness-only option
Sharpens the luma signal and leaves colour alone, so edges don’t pick up red or green fringes.
100% detail view
Inspect and pan at actual pixels, where sharpening decisions should be made.
Threshold protects smooth areas
Skin, sky and out-of-focus backgrounds stay clean while real edges get crisper.
Full resolution and batch
Download one image, or apply the same settings to many and save them as a ZIP.
Transparency respected
Edges next to transparent pixels are measured with opacity-weighted colours, so cut-outs don’t get dark rims.
The unsharp mask formula
Despite the name, an unsharp mask sharpens. It comes from darkroom printing, where a slightly blurred copy of the negative was sandwiched with the original to exaggerate edges. Digitally it takes three steps: blur a copy with a Gaussian of standard deviation σ (the radius), subtract it from the original to isolate the detail, and add that detail back scaled by the amount:
out = in + amount × (in − blur(in, σ)), applied only where |in − blur| ≥ threshold.
Example: an edge pixel of 140 whose surroundings blur to 120 has a detail value of 20. At 150% it becomes 140 + 1.5 × 20 = 170, while the darker side of the edge is pushed down in the same way, so the edge gains contrast. A pixel equal to its blurred surroundings has no detail and stays put. With Sharpen brightness only, the difference is measured on luma (0.2126 R + 0.7152 G + 0.0722 B) and the same offset is added to all three channels, so hues don’t shift.
Choosing amount, radius and threshold
Radius should roughly match the width of the detail you want to emphasise: fine hair and small text need less than 1 px, larger structures more. Threshold is counted in levels on the 0–255 scale; 2–5 stops noise and grain from being amplified in smooth areas. Good starting points:
| Use | Amount | Radius | Threshold |
|---|---|---|---|
| Web images after downsizing | 100–150% | 0.3–0.6 px | 0 |
| Portraits | 60–100% | 0.8–1.2 px | 3–6 |
| Landscapes, architecture | 100–150% | 1–1.5 px | 1–3 |
| Printing at 300 PPI | 150–200% | 1.5–2.5 px | 3–4 |
| Local contrast (“clarity”) | 20–40% | 8–10 px | 0 |
Common sharpening mistakes
- Halos. Bright and dark outlines along edges mean the amount or radius is too high. Reduce the radius first.
- Sharpening noise. Grainy low-light photos only get grainier; raise the threshold.
- Sharpening before resizing. Resize with the image resizer first and sharpen the final size — shrinking afterwards throws the added crispness away.
- Judging at the wrong zoom. A reduced preview hides sharpening; use the 100% view.
- Expecting miracles. Sharpening raises edge contrast. It cannot recover detail lost to heavy motion blur or a missed focus, and it is no substitute for a well-exposed, well-focused shot.
- Doing it twice. Many phones already sharpen JPEGs. Add a little, compare, and stop early — or soften overdone shots with Blur Image at a 1–2 px radius.
Sharpen Image FAQ
How do I sharpen a blurry picture online?
Add the picture, choose the Standard preset and look at the 100% view. Raise the amount until edges look crisp, lower the radius if halos appear, then download. Very blurry photos can only be improved a little.
What does threshold do in an unsharp mask?
It is the smallest difference, in 0–255 levels, between a pixel and its blurred surroundings that gets sharpened. Higher values leave smooth areas and fine noise untouched.
What radius should I use to sharpen an image?
Under 1 px for web-sized images and fine detail, about 1–2 px for full-resolution photos and prints, and 8–10 px with a low amount for a local-contrast “clarity” effect.
Why do I get white outlines after sharpening?
Those are halos from too much amount or too large a radius. Reduce the radius first, then the amount. Sharpening brightness only also prevents coloured edges.
Can I sharpen several images at once?
Yes. Add them all, set the sliders once and press “Download all as ZIP”. Each image is sharpened at full resolution.
Is the sharpen image tool free and private?
Yes. It is free with no sign-up or watermark, and images are processed in your browser, never uploaded.