JPEG compression — Verify the result

Worked example

To verify a compressed JPEG, compare bytes and dimensions with the original, then inspect gradients, text and faces at 100 percent.

After you press Compress image, the summary line lists the original and the result, for example a 4032 × 3024 JPEG at 4.2 MB followed by a 4032 × 3024 JPEG at a smaller figure. The saved file is named like kitchen-compress.jpg. The dimensions should be identical in both, because compression only changes the encoding, and a changed size means you ran another tool.

If the status line says the original was kept because it is smaller than the browser re-encode, no new file was made, and what you download is your source. That is a correct outcome for an already well-compressed JPEG. Try a lower quality value, or accept that this photo has little left to give.

Open the new file next to the original at the same zoom. Check the three spots you noted earlier, which are a flat wall or sky for banding, a sign or label for ringing halos around letters, and a face for smeared skin texture. Block edges or halos that were absent before mean the quality was set too low for this photo.

The usual error is accepting the number on the slider instead of the real figure. Compare the byte size against your target, such as a listing cap, and then repeat once if you are above it. Also confirm that the file opens and rotates correctly in the viewer or form where it will be used.

JPEG photograph; original retained

JPEG export; compare bytes and edges

A smaller file is only a success if the same spots you inspected on the original still read cleanly.