JPEG file format
JPEG is the format photography settled on: it throws away detail your eye barely registers and turns a 30 MB camera frame into a 3 MB file nothing refuses to open.
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What is JPEG?
JPEG was finalised in 1992 by the Joint Photographic Experts Group, and its method has barely changed since. The encoder cuts the image into 8×8 pixel blocks, converts each block from pixels into frequencies with a discrete cosine transform, and then rounds the high frequencies — the fine, rapid changes — down towards zero. Smooth areas survive intact; the sharpest micro-detail is quietly discarded.
That is why JPEG is superb for photographs and poor for screenshots. A photo is mostly gradual tonal change, exactly what the transform encodes cheaply. A screenshot is full of hard edges: black text on white, one-pixel borders, flat panels of colour. Those edges are high frequency, the encoder mangles them, and you get the grey halo known as ringing or mosquito noise around every letter.
JPEG also has no alpha channel. Nothing in the format can say "this pixel is transparent", so a logo exported from a design tool to JPEG arrives with a solid white or black rectangle behind it. That single limitation, plus its lossy nature, is why PNG and WebP exist alongside it rather than being replaced by it.
| Extension | .jpeg |
|---|---|
| Category | Image |
| MIME type | image/jpeg |
| Released | 1992 |
| Developed by | Joint Photographic Experts Group |
| Compression | Lossy |
| Transparency | No |
| Animation | No |
JPEG: frequently asked questions
- Does re-saving a JPEG make it worse each time?
- Yes, if you actually re-encode. Every save runs the lossy step again on an already degraded image, and the artefacts compound — a picture edited and saved twenty times looks visibly mushy. Simply copying, moving or renaming the file changes nothing; only opening and exporting again costs quality.
- What does the quality slider actually mean?
- It is not a percentage of anything. It scales the quantisation table that decides how aggressively frequencies are rounded away. Quality 90 is visually indistinguishable from the original for most photos, 75-85 is the normal web setting, and below 60 blocking artefacts start appearing in skies and skin tones.
- Why does my JPEG look terrible for a screenshot?
- Because screenshots are sharp-edged graphics, not photographs. The discrete cosine transform spreads a hard black-to-white edge across a whole 8×8 block, leaving smudges around text. Save screenshots, diagrams and logos as PNG or WebP lossless and keep JPEG for camera images.
- Can a JPEG have a transparent background?
- No. The format has no alpha channel at all, so transparency is flattened to a solid colour on export — usually white. If you need transparency, use PNG, WebP or AVIF; converting a transparent PNG to JPEG permanently bakes in whatever background colour was applied.
- Why did my photo rotate itself after conversion?
- Cameras and phones often store the picture in sensor orientation and add an EXIF orientation tag telling viewers to rotate it. Some tools honour that tag, others ignore or strip it, so the same file appears upright in one app and sideways in another. Re-saving with the rotation physically applied fixes it everywhere.
- Is progressive JPEG better than baseline?
- For web use, usually yes. A progressive file loads as a whole blurry image that sharpens in passes rather than filling in top to bottom, and it is often slightly smaller as well. A few very old or embedded decoders only handle baseline, which is the reason baseline is still the safe default.