Blur Image

Three different reasons to blur, three different results

Blur is used for at least three distinct purposes and they are not interchangeable. Depth of field blurs the background of a photograph while keeping a subject sharp, and it only looks right when there is a clear subject and background separation. Background removal blur flurs everything except a cut out subject. Privacy blur covers a specific area, such as a face, a plate, or a screen. A global blur over an entire image is the least useful of the four and usually just looks like a mistake.

NoteA global blur is almost never what is wanted. Blur one part of the image to make another part read better.

Radius, and why a heavy blur looks fake

Blur radius is how far the tool samples from each pixel to find the replacement value. A small radius softens detail slightly, which looks natural. A large radius mixes colours across a wide area, and because neighbouring pixels with very different colours get averaged together, the result looks like frosted glass rather than an out of focus lens. Real photographic blur is optically smooth and avoids mixing unrelated colours across a hard edge, which is why a large digital blur is immediately recognisable as artificial.

• A small radius looks natural, like a slightly soft lens.
• A large radius averages unrelated colours and looks frosted.
• Real lens blur is smooth and avoids hard edges being smeared.
• Blurring an edge between a face and background too heavily creates haloes.

NoteIf a heavy blur looks like frosted glass rather than an out of focus photo, the radius is too large.

Blurring for privacy does not reliably hide anything

This matters enough to state plainly. A blur is a reversible operation on the pixel data, and a person with the original image can undo a blur, and there are automated and manual techniques for recovering detail from a blurred region. A pixelated or heavily blurred face is much harder to recover than a lightly blurred one, but neither is a guarantee. If a face, address, licence plate, or document must genuinely not be identifiable, the reliable answer is to remove it entirely, not to obscure it.

NoteA blur is obscuring, not deleting. If it must not be recoverable, cover it or crop it out.

Blur and the boundaries of the edit

A blur applied near a hard edge, such as the boundary between a subject and the background, produces a visible halo, because the blur samples across the edge and mixes the two. It is the single most common flaw in a background blur. Reducing the radius, or leaving a gap between the blurred region and the edge, reduces it. The same issue applies at the image border, where sampling outside the image produces a dark or light rim unless the tool handles the edge properly.

• A blur crossing a hard edge creates a halo. This is the most common flaw.
• Reduce the radius, or leave a gap between the region and the edge.
• Blurring right to the image border can produce a rim artefact.
• Check the edges of the blurred area at 100 percent, not zoomed out.