JPG quality, measured

Two pictures from this site: the photograph on the front page (1800 × 900) and a screenshot of the front page's top (1366 × 900, text, buttons and a photo). Each was encoded by Chromium's own JPEG encoder — the one this site's Browser setting uses — at quality 95, 82 and 60, decoded again, and compared with the original over every pixel. The crops below are 60 × 40 pixels from the busiest part of each picture, enlarged four times without smoothing.

From 95 to 82 the file is 44% smaller and the crop barely changes; from 82 to 60 it shrinks another 39%, and fine grass starts to merge into eight-pixel blocks. That curve — big savings first, visible damage later — is why 82 is this site's Standard preset.

Even at 95 the screenshot measures 7 dB below the photograph: hard edges and flat colour are exactly what JPEG is worst at, and the browser encoder also stores colour at quarter resolution. For screenshots, a PNG is often the better file; when it must be a JPG, the Precise encoder with 4:4:4 subsampling helps — see chroma subsampling.

The converter's Measured mode does this measurement on your own files: each one gets the lowest quality that still keeps 40 dB (or 36 or 44, as you choose), found by encoding and decoding it. A plain sky ends up lower than a gravel path, and both look the same to the eye. The size curve in the Quality panel shows the trade for the selected photo before anything is saved.