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Photography Tutorial - Image Compression

If you've had a digital camera for more than a couple of days, you've probably come across the acronym JPEG by now, and you may be wondering exactly what it means. The letters themselves stand for the Joint Photographic Expert Group, a body of scientists, programmers and engineers from the camera industry who got together several years ago to come up with a new standard for file storage that would allow images from different computer programs to be interchangeable, so that a picture from one machine could be viewed on another without having to go through tedious file conversion programs.
The JPEG standard also happened to be an ideal format for storing pictures on a digital camera, because it uses something called file compression. This is a technique that allows a large number of images to be stored in a relatively small amount of memory by squashing the files so they take up less room. For this reason JPEG has become the standard image file format for all digital cameras.

As a gross simplification for the purposes of comprehension, JPEG file compression works by analyzing an image pixel by pixel, and determining how much the colour and brightness changes between one pixel and the next. If the change is below a certain threshold value, then the two pixels are considered to be the same. The data for the second pixel can therefore be discarded, saving storage space. This is done over the entire picture in a fraction of a second thanks to high-speed image processing.
For example, in this beach scene, a large area of the sky is roughly the same hue and shade of blue, so most of the data for this area can be discarded, saving a lot of file space. The beach and the sea have more detail, which means more variation in colour and brightness, so less information can be discarded. Of course discarding information means that fine detail and smooth tonal variations are lost, and strong image compression can have a very detrimental effect on image quality.
This is one of the reasons that professional photographers will prefer a camera that can shoot in Raw format. Raw images are stored in their uncompressed form, which makes the file sizes very large (as much as 50MB each for the top DSLRs) but of much greater quality. There are other uncompressed formats in common use, the most popular being TIFF, or Tagged Image File Format.

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