JPEG (Joint Photographic Experts Group)
The format that made sharing photos on the internet possible, shrinking images to a fraction of their original size.
| Full name | Joint Photographic Experts Group |
| Extension | .jpeg |
| MIME type | image/jpeg |
| Developer | Joint Photographic Experts Group (ISO/IEC working group) |
| Released | 1992 (ITU-T T.81); formally ISO/IEC 10918-1 in 1994 |
| Type | Lossy raster image |
| Compression | Discrete Cosine Transform (DCT) + Huffman coding |
| Color depth | Up to 24-bit (8 bits per channel, RGB or YCbCr) |
What is a JPG file?
JPEG is a lossy image format designed for compressing photographs and natural scenes. It shrinks file sizes dramatically by discarding visual detail the human eye is least likely to notice. The result is small, shareable files that still look good on screen.
JPEG stores images as a grid of pixels using lossy compression, meaning some image data is permanently removed during encoding. It works best on photos with smooth color gradients and tonal ranges. It is not well-suited to flat illustrations, text, or images that need sharp edges, because the compression introduces visible artifacts around high-contrast areas. Despite its age, JPEG remains the most widely used image format on the web and in digital cameras.
History
The Joint Photographic Experts Group was formed in 1986 as a collaborative working group under ISO and IEC. The group published the first JPEG standard in September 1992 as ITU-T Recommendation T.81, and it was later ratified as ISO/IEC 10918-1 in 1994. Digital cameras and the early web adopted JPEG rapidly through the mid-1990s, and it became the default format for photographic images worldwide.
How it works
A JPEG file is organized as a sequence of byte-aligned segments, each identified by a two-byte marker starting with 0xFF. The encoder splits the image into 8x8 pixel blocks and applies the Discrete Cosine Transform (DCT) to each block, converting pixel values into frequency coefficients. Those coefficients are then quantized (rounded) to reduce precision, which is where the size reduction and quality loss occur. Finally, Huffman coding compresses the quantized values into the final bitstream.
What it is used for
- Storing and sharing digital photographs from cameras and smartphones
- Publishing product and editorial photos on websites
- Sending photo attachments by email where file size matters
- Social media uploads where platforms re-compress images anyway
How to open it
Every major image viewer, web browser, and photo editor opens JPEG files without any extra software. Apps like Windows Photos, Apple Preview, Adobe Photoshop, GIMP, and any modern browser all support .jpeg natively.
Pros and cons
Strengths
- Very small file sizes for photographic content
- Supported by every browser, camera, and image editor in existence
- Adjustable quality setting lets you trade size for visual fidelity
- Works well for any image with complex colors and gradients
Trade-offs
- Lossy compression permanently discards image data on each save
- Visible blocking artifacts appear at low quality settings
- No support for transparency or alpha channels
- Poor quality for text, line art, and images with sharp edges
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JPG FAQ
What is the difference between .jpg and .jpeg?
There is no difference. Early Windows systems limited file extensions to three characters, so .jpg became the common shorthand. Both extensions refer to the same format and use the same MIME type, image/jpeg.
Does saving a JPEG multiple times reduce quality?
Yes. Each time you open and re-save a JPEG, the encoder applies lossy compression again. Quality degrades with each generation. If you need to edit an image repeatedly, work in a lossless format like PNG or TIFF and export to JPEG only for the final version.
When should I use JPEG instead of PNG?
Use JPEG for photographs and images with many colors and gradual tonal changes. Use PNG for screenshots, logos, illustrations, or anything that needs a transparent background or sharp text. PNG is lossless; JPEG is not.
What quality setting should I use when saving a JPEG?
For web use, a quality setting between 75 and 85 (on a 0-100 scale) gives a good balance of file size and visual quality. Settings above 90 produce large files with little visible improvement. Settings below 60 often show noticeable blocking artifacts.