Definition: .JPG files; JPEG Format

Definition: .JPG file; JPEG | Glossary entry

*.JPG file; JPEG

The *.jpg file is a strong contender for the best known file format. Of all the images that are published on the Internet today (2017), the *.jpg file is probably the most published. In terms of camera output, it is also the most frequently used. Most consumer cameras output the *.jpg file as the default. If you want to use another format, like RAW files, you usually have to deliberately select that option. Many of the consumer cameras on the market do not even produce RAW files. Instead, the *.jpg file is the format of choice. Why is that?

The *.jpg file format is small

From its popularity today we can assume that the standard has stood the test of time. So, what are the attributes of this format that make it so popular?

Image files are made up of a lot of data. When the *.jpg file standard was drawn up, large files would quickly use up drive space. On the early Internet, large files could take hours to download. Today,  faster Internet speeds, and larger hard drives are available. However, large files are more difficult to handle. So, a standard that reduces the size of the file to a usable and quickly transmitted size is still important.

In the coding of a *.jpg file is an algorithm that allows for the original file size to be reduced without apparent loss of image quality. In effect, the algorithm retains some data and dumps the rest – thus…

  • Data that is used for the visual representation is retained.
  • Background data that might be used in editing the image is dumped.

This type of data reduction in a file format is called a lossy format. Creating file in a lossy format means a file can be significantly smaller than a RAW file. Reductions of 60% are common. Yet, the file will retain a good quality visual effect. Further compression is possible, but it may damage the visual quality.

Problems with the *.jpg file lossy format

The use of a lossy format excludes a lot of manipulation of the image. Editors are most effective when they have all the original data available from the image sensor.

Picture it like this…

  • A dolls house with all the furniture and interior detail will delight the child and allow for a huge number of play scenarios.
  • A RAW file, packed with data, can also be manipulated in a large number of ways.


  • A model house without the interior fittings and detail looks impressive from the outside. But, the poor child gets no joy, or access, to its interior.
  • A *.jpg file contains only the essential, visible image data. The photos look good. However, for the user, there is poor editing flexibility. The interior or background data needed to support image changes is discarded.
*.jpg popularity

Small file sizes, easy storage and rapid transmission over the Internet made the *.jpg file popular. In general, keen amateurs and experts see the lack of editing ability as an inconvenience. After all, most keen amateurs and experts take the time to use the RAW format. Then later, after editing, they create the *.jpg file for display online. For most other photo users the file format is ideal.

Today, camera users want quick, easy visual results. Hence, the modern revolution in simple, cheap, image-making technology. So, consumer cameras use the *.jpg file. Users get a great image, produced straight-out-of-camera. The ease of use makes the *.jpg file format very attractive. Sales-seeking manufacturers therefore produce equipment and matching file format that make it easy for the user.

The JPEG standard

The Joint Photographic Experts Group (JPEG) is an organisation dedicated to this image coding standard. It is a working group of standards organisations who focus on the *.jpg file format. The JPEG standard (ISO/IEC 10918), created in 1992 (updated 1994) is the format which creates the humble *.jpg file.
The JPEG standard (ISO/IEC 10918-1 | ITU-T Recommendation T.81) is actually a five-part specification. These parts can be broadly defined as…

  1. The core coding technology :: what creates the *.jpg file.
  2. Compliance testing :: This ensures it meets the JPEG standard.
  3. Extensions to the coding standard :: This allows various types of code to meet the standard.
  4. JPEG profiles :: These allow for colour profiles, compression types; metadata tag types; etc.
  5. JPEG File Interchange Format (JFIF) :: Shows how files my be transmitted or modified for transport over the Internet.
The *.jpg file overall

The *.jpg file format has proven both robust and flexible. It has probably been the format behind more pictures than every other image format including printed. People like you are uploading literally billions of images on the Internet every day. The ability for the file to retain both high quality image replication, and low size duplication gives it great advantages. That continues to be the case as hard drives get bigger and storage gets cheaper.

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Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photographer and editor of this site. He has also run a major website, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.
By Damon Guy.


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