Bit Depth

Computers are actually very dumb machines. All they can recognize are two opposite states—on and off, or zeroes and ones. What makes them appear to be smart is that they can read, use, sort, alter, and output these bits of data so blindingly fast.

In digital photography, the number of colors that can be saved in a picture is measured by its bit depth—how many bits are devoted to each primary color (or color channel). The following table shows the number of colors available at various bit depths.

Bit Depth Number of Colors

8 256

10 1,024

12 4,096

14 16,384

16 65,536

A grayscale image has 8 bits for the single color of black, which means it has 256 levels of gray from black to white. However, the typical digital color photo has three primary colors: red, green, and blue (RGB). And most digital photos (once they are saved and brought into the computer) have 8 bits of color (different hues or shades) associated with each of these three channels (which is what makes it a 24-bit image—3 primary channels x 8 bits each). To calculate the number of colors possible in the full-color RGB image, multiply the number of colors for each primary color. That translates into 256 x 256 x 256 colors forthe 24-bit RGB—or approximately 16.7 million possible colors.

The human eye is capable of distinguishing about 12 to 14 million colors, so 24-bit color is considered the minimum for photo-realistic images (also called continuous tone, because the eye can't perceive any changes in transitional gradations from one color or shade to another). Of course, just because you have the ability to capture millions of colors doesn't mean that's whatyou'll get. A typical photo may have about 5,000 different colors, but that's from an enormous palette of possibilities that adds up to millions or even billions.

What makes bit depth important isn't just the number of colors, but the gradual, seamless gradations between colors that are possible when you have more colors. Imagine a portrait photo. The face should appear to have soft transitions among the many shades of natural skin tones. Otherwise, you would see unnatural hard edges between colors, which would show up as banding (posterization) or lines. Color Figure 6 (in the color inserttoward the back of the book) clearly demonstrates the difference between 24-bit color with its photo-realistic gradations and 8-bit color posterization.

Inside the digital camera (especially in more expensive professional models), photo data can actually be 16 bit, which adds up to 48-bit depth (billions of different possible colors) for an RGB image. You can't really use all that data for any useful output (print or screen), but Photoshop and a handful of other professional programs can open and edit such large files. Then, the photographer can choose

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