What "quality 80" actually throws away

The quality slider on every image compressor goes from 1 to 100, which makes it look like a percentage. It is not. It is an index into how aggressively detail gets discarded, and the relationship between the number and what you see is deeply non-linear.

What lossy compression actually does

Lossy image compression rests on one observation about human vision: we are far more sensitive to brightness than to colour, and far more sensitive to broad shapes than to fine detail. So the compressor throws away the things you are least likely to notice.

It happens in roughly three stages.

Colour gets thinned out first. The image is split into a brightness channel and two colour channels, and the colour channels are stored at half resolution — sometimes a quarter. This alone removes a large fraction of the data, and on a photograph it is essentially invisible, because your eye reconstructs colour boundaries from the brightness edges that are still sharp.

The image is broken into blocks and described as waves. Each small block gets converted from "here are the pixel values" into "here is how much coarse variation, how much medium variation, how much fine variation". Most real image blocks are mostly coarse variation — that is what a smooth gradient is.

The fine detail is rounded off. This is where the quality setting lives. It controls how coarsely those variation numbers get rounded. Round gently and you keep almost everything. Round hard and the fine-detail numbers all become zero — and a block whose fine detail is zero is a flat patch of colour.

Why 80 is the number everyone lands on

Between roughly 95 and 80, you are discarding detail that was genuinely below the threshold of perception, and the file shrinks a lot for it. A photo at quality 80 is typically 60–80% smaller than the same photo at 100, and put side by side, most people cannot pick which is which.

Below about 60, the rounding starts zeroing out detail that was doing real work. You see it first in the places with the most subtle gradation: skies band into visible steps, skin goes waxy, out-of-focus backgrounds develop a faint crawling texture.

Below 40, the block structure itself becomes visible. The 8×8 grid the compressor works on stops being hidden and you get the familiar mosaic.

So the curve is: a lot of saving for nothing between 100 and 80, a reasonable trade between 80 and 60, and a bad deal below that.

Why your screenshot fell apart at quality 90

Everything above assumes a photograph. Images with hard edges behave completely differently, and much worse.

A hard edge — black text on white, a logo boundary, a UI border — is, in the compressor's terms, an enormous amount of fine detail concentrated in one place. It is exactly the thing the rounding step is designed to discard. So the compressor tries to reconstruct a razor-sharp edge from the coarse components it kept, and gets ringing: faint light and dark bands echoing outward from the edge.

On text, this shows up as a grey haze around every letter. It is visible at quality 90 and obvious at 75, on an image where a photo would still look perfect.

The colour thinning makes it worse. Red text on a white background loses most of its colour resolution in the first stage, and the edge smears.

This is not a fixable setting. It is what the format is. Screenshots, logos and diagrams need a lossless format — PNG, or lossless WebP — and no quality slider at all.

Compression is not reversible, and it accumulates

Every time you open a lossy image and save it again, it goes through the whole process afresh — including on the artefacts introduced last time, which are now treated as real image content and given their own artefacts.

Save a JPG ten times at quality 80 and it looks noticeably worse than saving it once at quality 40, despite the "higher" setting. This is generation loss, and it is why the workflow matters more than the number: always compress from the original, never from a compressed copy.

It is also why "converting to PNG to fix the quality" does nothing. PNG will faithfully preserve every artefact already baked in.

Target-size mode, and what it is doing

Compressors that let you specify a file size are not doing anything clever — they compress at one quality, check the result, adjust, and try again until they land near your number. It is a search, run a handful of times.

It is genuinely useful when you have a hard limit to meet, like an upload cap. But it is worth knowing that it will happily blow past the point where the image looks bad in order to hit your number. If the result looks wrong, the target was too small for that image, not a fault in the tool.

Practical settings

• Photographs for the web: 75–85. Start at 80.
• Large hero images: you can go lower, 65–75 — they are viewed briefly and scaled.
• Product photos where detail matters: 85–90.
• Anything with text, lines or flat colour: not lossy at all. Use PNG.
• Archival or source files: keep a lossless original somewhere and compress copies from it.