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Tutorial: Digital Image Bit Depth

Scanning Technologies: Bit DepthIf you have shopped for either a scanner or a scanning service, chance are good that you have stumbled onto mention of a mysterious piece of techno babble: "bit depth". What is bit depth anyway?

"Bit depth" refers to the number of bits a computer uses to represent each pixel of an image in order to determine color. Most images are "8-bit" images. In an 8-bit image, each channel (red, green, and blue) of each pixel is described by an 8-bit number. Because a bit can be either "on" or "off", a 1 or a 0, each channel can have up to 256 different shades. This gives a grand total of 16 million different combinations when we look at all 3 channels.

Sometimes 3-channel 8-bit (red, green, and blue) is called 24-bit color because each channel has 8 bits (and 3 times 8 is 24). It can also be called "true color" because the 16 million colors so accurately represents the colors seen in the outside world. The human eye generally can't distinguish more than that amount. 24-bit color digital images are what we see everyday in "normal" images we view on the web and those that come out of most digital cameras (high end cameras excluded).

Images with a higher bit depth may store more than 256 color shades for each channel. For example, 16-bit images have 65,536 different shades per color channel -- 256 times more data than is in the 8-bit image. The higher bit depth does not increase the overall resolution of the image; the same number of pixels is present in the image. Rather, each pixel can more finely distinguish colors that are very close together. For most images, higher bit depth rarely has a noticeable effect on the image, but it can have a dramatic increase the file size. Not all photo editing and viewing programs understand higher bit-depth images (though this is changing). Additionally, most photo printing services will not work with anything other than 24-bit (standard bit depth) images.

So why have higher bit depth images? The major advantage to including higher bit depth to your image is that when using photo-editing techniques there is more data with which to work. When editing photos, you can never add data (i.e., detail) to the image, only take it away. High-bit depth images have more data to "throw away" when editing without the loss becoming noticeable. Many times these "problems" can only be made obvious with aggressive modification of the digital images (though there are examples where very simple changes cause problems). Frequently, problems with heavy editing manifest themselves when banding (the technical term for rough color transitions) appears in broad areas of roughly uniform color, for example in the blue sky.

If you have had us scan your film or negatives, you may have noticed that we normally scan images at 8 bits per channel. Why not scan at a higher bit depth? For most of our customers and for most uses of the resulting images, scanning at higher bit depth does not increase image quality, but it does increase scanning costs and makes the resulting digital files much more cumbersome to handle because of their size and because of problems using the resulting images with some photo editing and viewing programs. We can do scans at 16 bits per channel if you have a need - just ask!

Questions or comments? Please let us know.