What the quality slider actually changes, format by format

2026-09-16

Every image tool has a quality slider, and most people use it the same way: drag until the file is small enough, hope nothing visible broke. It behaves quite differently depending on the output format — knowing which is which makes the number mean something.

JPG and WebP: classic lossy quality

Here the slider is the traditional trade: lower numbers discard finer image detail to save bytes. Our presets are chosen around practice — 82 "Recommended" sits below most camera defaults (re-encoding above the source quality only preserves old artifacts in loving detail, as an earlier post explains), 62 "Smaller" for chat-grade images, 94 "High quality" when the image will be inspected closely.

PNG: the slider picks a palette, not a quality

PNG is lossless, so there is no detail dial to turn. Instead, the slider chooses how many colours the palette compressor may keep: the top of the range keeps 256, the middle 128, the bottom 64 — with dithering to keep gradients smooth. And if your image already has few enough colours, the reduction is exact and nothing is lost at all. Details in the PNG post.

PDF: the slider reaches inside the page

When you convert an image to PDF here, the picture is stored JPEG-style inside the page — so the same quality number applies. A receipt at "Recommended" makes a compact document; a portfolio page deserves "High quality".

BMP and TIFF: the slider does nothing — on purpose

Both formats are written uncompressed, so there is no quality to trade and the file size is fixed by the pixel dimensions alone. If a tool shows you a quality knob affecting a BMP, something else is quietly changing — usually the dimensions.

What we deliberately don't do

We don't auto-tune quality per image to hit a target size. A slider you set is a decision you made; an optimiser guessing on your behalf is how files come back looking worse than expected with no way to ask why.

The short version

• JPG/WebP: real lossy quality — 82 is the sane default, don't exceed the source.
• PNG: the slider sets palette size (256/128/64), lossless when colours already fit.
• PDF: applies to the JPEG stored inside the page.
• BMP/TIFF: uncompressed — the slider genuinely does nothing.