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Joint Photographic Experts Group image · .jpg

The JPG (JPEG) format

The format photographs live in. Lossy block-based compression trades detail you are unlikely to notice for files small enough to send.

JPEG is a compression method designed around human vision: it discards the image information our eyes are worst at noticing, particularly fine colour variation. On a photograph that is an excellent bargain. On a screenshot of text it is a bad one, and the difference is visible in a second.

The format itself

What JPG actually is

The Joint Photographic Experts Group published the standard in 1992. Strictly, JPEG is the compression scheme rather than the file layout — what you download as a .jpg is usually a JFIF or Exif file wrapping JPEG-compressed data, which is how cameras manage to store shutter speed, lens and GPS alongside the picture.

Compression works by converting the image to YCbCr — one brightness channel and two colour channels — then usually storing the colour channels at reduced resolution, a trick called chroma subsampling that exploits how much sharper our perception of brightness is than of hue. Each channel is cut into small blocks, each block is transformed with a discrete cosine transform into frequency coefficients, and the high-frequency coefficients are quantised away. The encoder quality setting, normally expressed as 0 to 100, controls how aggressively that quantisation happens.

The consequence is that JPEG is lossy in a compounding way. Decoding and re-encoding a JPEG requantises coefficients that were already approximations, so each save-and-resave cycle degrades the image further — generation loss. Nothing in the format records the original, so no tool can recover what an earlier save threw away. It is also strictly a flat, single-image format: 8 bits per channel, no layers, no alpha channel and therefore no transparency of any kind.

Where it is the right answer

Common uses

  • 01

    Photographs from cameras and phones, which is what the format was designed around

  • 02

    Product and editorial imagery on websites where a small file matters more than pixel-exact fidelity

  • 03

    Email attachments and messaging, where photo dimensions are large but bandwidth is not free

  • 04

    Page previews and thumbnails generated from documents for quick visual scanning

  • 05

    Photographs of documents captured on a phone, usually as a step before OCR or PDF assembly

Both sides

What JPG does well, and where it stops.

Strengths

  • Very small files for continuous-tone photographic content, at quality most viewers cannot distinguish from the original
  • Decoded by effectively every device, browser, printer and photo application in existence
  • The quality setting gives direct, predictable control over the size-versus-fidelity trade-off
  • Carries Exif metadata, so camera settings, orientation and timestamps travel with the image
  • Fast to encode and decode, including in hardware on cameras and phones

Limitations

  • No transparency. There is no alpha channel, so anything that needs a cut-out background cannot be a JPEG.
  • Generation loss is cumulative: every re-save degrades the image again, and the damage is permanent.
  • Poor on sharp edges. Text, screenshots and line art pick up visible ringing and mosquito noise around high-contrast boundaries.
  • Limited to 8 bits per channel, which is not enough headroom for heavy tonal editing or high-dynamic-range work.
  • Quality is an encoder setting, not a property of the pixels — a 4000-pixel-wide image saved at low quality is large and still soft.

Specification

Technical detail

The facts worth knowing before choosing JPG for a job, rather than after.

Full name
Joint Photographic Experts Group image
MIME type
image/jpeg
Extensions
.jpg, .jpeg
Standardised
1992, by the JPEG committee
Container
Usually JFIF or Exif wrapping the JPEG data
Compression
Lossy, block-based discrete cosine transform
Colour model
YCbCr, commonly with chroma subsampling
Bit depth
8 bits per channel
Transparency
Not supported

Tools that apply

Converting JPG in your browser

Each listed MyPDFilles conversion runs on your device: the file is read from your disk, converted in the tab and handed back as a download. Educational references to external software are not covered by that claim.

Questions about JPG

Is there any difference between .jpg and .jpeg?

None at all. They are the same format with two spellings of the same extension — the three-letter form survives from filesystems that allowed no more than three characters. Both decode identically and neither is more compatible today.

Does re-saving a JPEG at 100 quality avoid losing anything?

No. Quality 100 still quantises coefficients and still re-encodes what was already an approximation, so detail is lost — just less of it, in a much larger file. If you need to edit repeatedly, work in a lossless format and export a JPEG only at the end.

Why does text look so bad in a JPEG?

Sharp edges are exactly the high-frequency detail the compression discards first. Around black-on-white letterforms this appears as faint ripples and grey speckling. For screenshots and anything containing type, PNG is the right choice.

Can I make a JPEG background transparent?

Not in the format itself — there is no alpha channel to hold the information. You would need to save as PNG or WebP, both of which support real per-pixel transparency.

Useful background

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