Digital photos are stored as files on memory cards and computer hard disks. They can be stored in different file formats (each format has a unique file extension). Digital photo files can be compressed to save storage space and can use a standard or a proprietary format.
Digital photos are saved as digital files on electronic media. These digital photo files are a collection of bytes. JPEG is a standard file format for storing such digital photos. The JPEG format supports compression and allows very efficient high quality storage of digital photos. When using the JPEG format any software from different vendors that adheres to the standard can create and process JPEG files.
JPEG – Joint Photographic Experts Group
The JPEG file format was designed by a special industry group for the purpose of storing and compressing high quality digital photos. Later on JPEG became an ISO standard and it is probably one of the most popular formats for storing digital photos today. JPEG files are also known simple as JPG files (named after their common file extension .JPG).
The design goal of the group was the efficient storage of digital photographic files while maintaining their original high quality (or quality which is as close as possible to the original one). JPEG files can support lossless compression but since lossless compression is inefficient when compressing digital photos it is hardly used. In most cases JPEG files use lossy compression to efficiently represent digital photos.
It is easy to understand the motivation for such efficient compression by using an example: Digital photos files are built of pixels each one has a value representing its color and intensity. Each pixel occupies 3 bytes (one for Red, one for Green and one for Blue - representing the color and intensity of the pixel). If you shoot a photo using an 8 mega pixels digital camera the photo will have 8000000 pixels each one occupying 3 bytes. The total file size would be 8000000*3=24000000 or 24Mbytes. This is a very big file. Big files are hard to manipulate they take a long time to send by email, they occupy large storage space and they take longer to load. If that same file was compressed into a JPEG digital photo file its size would reduce to about 3Mbytes – a much more manageable file size.
JPEG files are extremely efficient in compressing digital photos. They are using compression methods that were specifically designed for digital photos and take advantage of digital photos attributes that allow a more efficient representation of pixels. The JPEG format standardized the way a digital photo is represented and suggested ways to compress digital photos. Using this standard software from one vendor can compress and create a JPEG digital photo file while software from a different vendor can read and process it. There are many different compressors implementations that differ in the quality of the compressed photo. Also – the digital photo compressor can be set to different levels of compression. When using a very aggressive compression setting the digital photo file can be very small – the side effect however is degradation in quality – most noticeable is the appearance of large square areas with the same color and pattern.
JPEG files are the most commonly used ones by digital cameras to store compressed digital photos on memory cards and computer hard disks as they result in small file sizes and hardly any noticeable photo quality degradation. Most cameras also let the user set the level of compression applied to the digital photos – usually three level are provided: low, medium and high – where low results in minimal file size reduction but superior quality and high results in the smallest files but the highest degradation in quality - in most cases however that degradation is hardly noticeable by the amateur photographer.
Danette Mckay explains about this subject in more depth at photo printing