How to sharpen a blurry PDF

What blur is

In a sharp page, the edge of a letter goes from dark to light across one or two pixels. In a blurry page it goes across many, so the stroke looks smeared. The edge information is still there but spread out, which is the part sharpening can steepen again.

Three kinds of blur, and what helps

• A slightly soft scan. Edges are a little wide and everything else is fine, typically from scanner optics or JPEG compression. Sharpening works well here.
• A low-resolution image. The letters are made of visible blocks because the image was small and was stretched to fit. Sharpening makes the blocks harder, not the letters clearer. Contrast and darkening still help readability.
• Motion or focus blur. The whole page is smeared in a direction or everywhere. Only a mild improvement is possible, and strong sharpening produces rough, artificial edges.

How sharpening works

The usual method is an unsharp mask. A blurred copy of the page is subtracted from the page to find the edges, and a controlled amount of that difference is added back. Where an edge is soft, adding it back makes the edge steeper.

Two things go wrong when it is done carelessly. Pushing a pixel lighter than the lightest pixel beside it, or darker than the darkest, draws a bright or dark outline around letters, called a halo. And amplifying small variations strengthens noise and compression artefacts. Good sharpening limits each pixel to the range of its neighbours, which prevents halos, and ignores variations below a small threshold, which protects against noise.

Steps

1. For an ordinary soft scan, open Sharpen PDF. It changes only the edges and leaves tone and colour as they were.
2. For a page that is clearly blurry and also pale, open Fix blurry PDF, which adds contrast and darker ink and lets you set how wide the blur is.
3. Start gentle. Zoom in on small text and drag the before/after slider.
4. If edges still look smeared, step up the strength or raise the blur size. If they look rough or grainy, lower the amount or raise noise protection.
5. Download the new PDF. The original is never modified.

Blur size: matching the sharpening to the page

Sharpening has a radius, the distance over which it looks for an edge. A page with only slightly fuzzy text needs a small one. A page smeared across many pixels needs a larger one. Too large on a sharp page makes it look rough, which is why the setting is exposed in Fix blurry PDF rather than hidden.

When to rescan instead

• You still have the paper and the scan is below about 200 DPI. Rescan at 300 DPI.
• The blur is motion or focus blur from a phone. Hold the phone parallel to the page, in diffuse light, and retake it.
• Small print is unreadable even on the strongest setting. Nothing further can be recovered from the file.