The JPEG file format for photos is popular because it has good compression characteristics and lets you set the compression amount when saving a JPEG image. One way to shrink the file size, then, is to change the compression ratio. Photoshop lets you do this by tinkering with the "Quality" value during its JPEG save process. Paint Shop Pro, another photo editing package, lets you choose a compression ratio value between 1 and 99. A level of 1 gives the lowest compression ratio and best image quality. The image quality goes down as the compression ratio number goes higher. Other programs may have different settings, but they will be employing the same underlying concepts. When the compression ratio is higher, the image becomes more pixelated; the program is throwing out more and more information in order to squeeze the file down to size. Reducing the image size instead can give you a smaller file size without pixelating the image.
One of the things you'll be employing, whether or not the software tells you about it, is a file-reduction ratio. The file-reduction ratio is a measure of how much a file's size is reduced in relation to its original size, for example 10:1 or 20:1. It's determined by several factors, including file size, file type and the compression scheme that's used. JPEG files will achieve somewhere between 1.1:1 and 50:1 [source: Florida State University].Text files, meanwhile, can often be reduced in half or more, because most languages make use of a lot of common letter and word combinations (compression succeeds or fails based on, among other things, how much redundant information it can throw out). If, on the other hand, the code making up a file like an MP3 doesn't feature a lot of repeating patterns, it can't be compressed very much. Some compression software, it should be noted, is specifically designed for certain file types, so it will be able to do a better compression job on those than on different types.
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