Sharpen Image

How to use the sharpen image

1. Choose an image.
2. Pick an effect.
3. Check for halos, then download.

About this tool

Each pixel is recalculated from itself and its eight neighbours, weighted by a small grid called a kernel. Sharpening weights the centre pixel heavily and subtracts its neighbours, which increases contrast wherever the image already changes — and that reads to the eye as sharper.

The important limitation: sharpening cannot recover detail that was never captured. A blurred photograph has lost information, and no filter puts it back. What sharpening does is exaggerate the edges that survived, which helps a slightly soft image and makes a genuinely blurred one look worse — grainier and edged with halos rather than clearer.

Those halos are the signature of over-sharpening. A bright line along one side of a dark edge and a dark line along the other means you have gone too far, and it is far more obvious in print than on screen.

Edge pixels use the nearest in-bounds pixel rather than wrapping to the opposite side, which would put the far edge's colours into the border — visible as a bright fringe along a dark photograph's edge.

The other kernels use the same machinery: box blur averages equally, emboss subtracts one diagonal from the other to fake relief, and edge detection keeps only where brightness changes.

Common uses

• Recovering a slightly soft photograph.
• Sharpening a scan before printing.
• Producing an edge or emboss effect deliberately.

Frequently asked questions

Can sharpening fix a blurry photo?

No. Blur destroys information and nothing recovers it. Sharpening exaggerates surviving edges, which helps a slightly soft image and makes a truly blurred one look worse.

What are the light and dark lines along edges?

Halos — the sign of over-sharpening. Ease off until they disappear, and check at full size rather than zoomed out.

Is my image uploaded?

No. The convolution runs on your device through the canvas.