PDF brightness vs contrast: which one to change

What each one does

Picture the page as two clusters of tones: the ink, which is dark, and the paper, which is light. Brightness slides both clusters up or down together. Contrast pulls the clusters apart or pushes them together. Readability depends on the distance between them, so only contrast changes how readable the page is.

Measured on a real faded scan

These results come from this site’s own engine, on its faded demo scan (a synthetic air waybill made faint on purpose). The numbers are the lightness of the ink (2nd percentile) and the paper (95th percentile) on a 0 to 255 scale, and the gap between them. They illustrate the method; they are not a benchmark of real documents.

• Original: ink 187, paper 231, gap 44.
• Brightness +30: ink 217, paper 255, gap 38. The paper hit white and could go no higher, the ink kept rising, and the gap shrank. The page is lighter and harder to read.
• Brightness −30: ink 157, paper 201, gap 44. The page is darker, but the gap is unchanged, so it is no more readable.
• Contrast ×1.5 around mid-grey: ink 216, paper 255, gap 39. Worse, not better.
• Contrast ×2 around mid-grey: ink 246, paper 255, gap 9. The page nearly disappears.
• Strong contrast (measured black and white points): ink 81, paper 255, gap 174.
• Clean Scan (background-focused): ink 176, paper 255, gap 79.

Why ordinary contrast fails on a faded page

A standard contrast slider pivots around mid-grey, 128. On a faded scan both the ink and the paper are above that pivot, so “more contrast” pushes both lighter, and the paper clips to white. The ink is dragged along with it.

The fix is to put the pivots where the page actually is: the black point at the measured ink level and the white point at the measured paper level, then stretch what is between them. That is what moved the ink from 187 to 81 above, and it is what the PDF contrast enhancer does.

Which control for which symptom

• Text looks faint or grey: contrast and text darkness. See Fix a faded PDF.
• The whole page looks dull, grey or dingy: brightness and background whitening. See Brighten PDF.
• One side is darker than the other: neither. That is uneven lighting; see Remove scan shadows.
• It looks fine on screen but prints faint: contrast and darkness in the file, plus the printer settings.

Where brightness does help

Brightness is the right control when the whole page sits too dark, for example an underexposed phone photo of a document, or a dark photocopy whose mid-tones have clogged. There it lifts the page without touching the ink’s separation from the paper. In the Brighten PDF tool it never moves pure black or pure white, so it cannot clip the ink or the paper.