JPEG compression — Troubleshooting

Worked example

JPEG problems come down to a file that barely shrinks, an original returned, banding, text halos or a file that will not load.

If the file barely shrinks even at a low setting, the photo was probably compressed hard before, or it is dominated by fine noise such as grain from a night shot. Noise is expensive for JPEG to store. Resize the photo to the size you really need, which cuts more than quality can.

If you receive the original back, the status line says the original was kept because the browser re-encode was not smaller. This tends to happen with already optimised files and at quality values near 95. Choose a lower value such as 70 to 75 and run it again.

Banding in a sky or a soft wall and halos around letters on signs are the two classic signs of too low a setting. Raise the slider by 5 to 10 points and retest instead of dropping it further. On a screenshot that is full of text, JPEG is the wrong format in the first place and a PNG or WebP will look cleaner at similar weight.

When the file fails to load, check that the browser can open it: a HEIC or very large photo may not decode, and an image above 64 million pixels is refused with a message to crop or resize first. Convert HEIC to JPG, or shrink the picture, and then compress. If the picture shows the wrong way up, check the original in an image viewer first, because the compressed copy carries no separate orientation tag.

JPEG photograph; original retained

JPEG export; compare bytes and edges

When quality 78 still leaves the file too large, reduce the dimensions rather than going below 60.