Web Design in a Nutshell, 3rd Edition

Variable Compression Levels

One advantage to JPEGs is that you can control the degree to which the image is compressed. The higher the quality, the larger the file. The goal is to find the smallest file size that still maintains acceptable image quality.

The quality of a JPEG image is denoted by its “Q” setting, usually on a scale from 0 to 100. In nearly all programs, the lower numbers represent lower image quality but better compression rates (and smaller files). The higher numbers result in better image quality and larger files.

For the most part, the Q setting is an arbitrary value with no specific mathematical significance. It is just a way to specify the image quality level you’d like to maintain. When JPEG compression goes to work, it compresses as much as it can while maintaining the targeted Q setting. The actual compression ratio depends on the content of the individual image.

The scales for specifying Q-settings (or “Quality”) vary among tools that create JPEGs. Most current web tools use a scale from 0 to 100; however, you will still find some that use a scale from 0 to 10 or 0 to 12. The numbers themselves are not significant (a 30 in one program may be radically different than a 30 in another); what matters is the way the image looks and its resulting file size.