How JPEG compression actually works
JPEG discards information the human eye is poor at noticing. It converts the image into frequency components and throws away the fine, high-frequency detail first, while preserving the broad shapes and colours your eye tracks. The quality setting controls how aggressively it discards.
Because our vision is much more sensitive to brightness than to colour, JPEG also stores colour at reduced resolution. This is why dropping from quality 100 to quality 80 often halves the file with no visible change, while dropping from 50 to 30 is immediately obvious.
Re-compressing an already-compressed JPEG
Every JPEG save is lossy, and the damage accumulates. Re-encoding the same JPEG repeatedly at middling quality produces the soft, blotchy look of an image that has been passed around too many times.
So work from the highest-quality original you have rather than from a file that has already been compressed once. One good compression pass from the original beats three passes from a copy — and if you need several output sizes, generate each one from the original.
When JPEG is the wrong format
JPEG is built for photographs — continuous gradients, soft transitions, natural texture. It does poorly on screenshots, text, logos and line art, where it smears sharp edges and leaves visible ringing artifacts.
For that kind of image, use PNG for compatibility or WebP for size. Both keep edges crisp.
Frequently asked questions
- Does compressing to 100KB reduce the pixel dimensions?
- Not unless you ask it to. By default the dimensions stay as they are and only the encoding quality changes. You can set explicit dimensions if you need both.
- Can I get the original quality back afterwards?
- No — JPEG compression is lossy and the discarded detail is gone for good. Keep your original file; treat the compressed version as a derivative.
- Why is my PNG so much bigger than 100KB?
- PNG is lossless, so it cannot trade detail for size the way JPEG does. Photographs saved as PNG are often 5–10× larger. Convert to JPEG or WebP for photos.