JPEG, which takes its name from the Joint Photographic Experts Group committee that developed and standardised the format, is one of the most widely used methods of image data compression in digital photography. It is designed to significantly reduce the file size of digital images by analysing and discarding visual information that the human eye is least likely to notice, making image files more manageable in terms of storage space and faster to transmit and share over networks and the internet.
JPEG uses a form of compression known as lossy compression, which means that in the process of reducing the file size, some image data is permanently discarded and cannot be recovered. The degree of compression applied - and therefore the amount of data lost - is typically controlled by a quality setting, usually expressed as a numerical scale or a descriptor such as low, medium, high, or maximum quality. At higher quality settings, less compression is applied, more image data is retained, and the resulting file size is larger but the image quality remains closer to the original. At lower quality settings, more aggressive compression is applied, file sizes are reduced further, but the image begins to exhibit visible compression artefacts - blocky, smeared, or blotchy areas, particularly noticeable in regions of fine detail, smooth gradients, and sharp edges - as a result of the greater data loss.
The compression process works by dividing the image into small blocks of pixels, typically eight by eight pixels in size, and applying a mathematical transformation known as the Discrete Cosine Transform (DCT) to each block. This process converts the pixel data into a representation based on frequency information, allowing less visually significant details to be identified and discarded while retaining the most perceptually important information. The result is then further compressed using lossless encoding before being written to the file.
An important characteristic of JPEG compression that photographers should be aware of is that the quality loss is cumulative. Each time a JPEG file is opened, edited, and resaved as a JPEG, another round of lossy compression is applied, resulting in a further reduction in image quality that compounds with each successive save. For this reason, photographers working on images that require multiple rounds of editing are advised to work in a lossless format such as TIFF or the camera's native RAW format throughout the editing process, converting to JPEG only as a final step when the image is ready for output, sharing, or web use.
Despite its lossy nature, JPEG remains the dominant file format for photographic images due to its excellent balance of file size reduction and image quality at higher quality settings, its near universal compatibility across devices, platforms, and software applications, and its suitability for web and screen display. It is the default capture format on the majority of digital cameras and smartphones, and is the standard format for images shared online and via email.