When a scanner converts something into digital form, it looks at the image pixel by pixel and records what it sees. That part of the process is simple enough, but different scanners record different amounts of information about each pixel. How much information a given scanner records is measured by its bit depth.

The simplest kind of scanner only records black and white, and is sometimes known as a 1-bit scanner because each bit can only express two values, on and off. In order to see the many tones in between black and white, a scanner needs to be at least 4-bit (for up to 16 tones) or 8-bit (for up to 256 tones). The higher the scanner's bit depth, the more accurately it can describe what it sees when it looks at a given pixel. This, in turn, makes for a higher quality scan.

Most color scanners today are at least 24-bit, meaning that they collect 8 bits of information about each of the primary scanning colors: red, blue, and green. A 24-bit unit can theoretically capture over 16 million different colors, though in practice the number is usually quite smaller. This is near-photographic quality, and is therefore commonly referred to as "true color" scanning.

An increasing number of manufacturers are offering 30-bit and 36-bit scanners, which can theoretically capture billions of colors. The only problem is that very few graphics software packages can handle anything larger than a 24-bit scan, because of limitations in the design of personal computers. Still, those extra bits are worth having. When a software program opens a 30-bit or 36-bit image, it can use the extra data to correct for noise in the scanning process and other problems that hurt the quality of the scan. As a result, scanners with higher bit depths tend to produce better color images.

One warning: not all monitors can display a 24-bit, true-color image. Many monitors (especially older ones) display only 8-bit images, with just 256 colors. If an image looks patchy or distorted on screen, it may be the fault of the monitor, not the scanner.