Most everything you work with relies on 8-bit images. Your monitor, your digital
camera, and so on usually rely on 8-bit values for each color. In other words, 256
brightness levels per color per pixel. Eight bits of data are used for each color that
makes up an image. In an RGB image, that means a byte for each color, or 256
brightness values for each color.
Three times eight is 24, which is the origin of the term 24-bit color. By default, most
cameras and scanners capture in 24-bit RGB, but this can be changed in your printer
options during scanning. This means that more than 16 million possible brightness
and color combinations are possible for each pixel. Although this is impressive, your
eye can actually take advantage of even more color.
184 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
EIGHT-BIT AND 24-BIT USED INTERCHANGEABLY
Often you will read about a 24-bit capture, and then the same image referred to as 8-bit.
The terms are related. Remember, a 24-bit image consists of three colors (RGB) at an 8-bit
depth (3×8 = 24). So, a 24-bit image or scan comprises three 8-bit channels of color.
12- and 16-bit Scan Rates
Every scanner and digital camera is capable of producing 24-bit images. Higher-end
scanners are capable of 36-bit and 48-bit scans. This works out to 12-bits or 16-bits
per color (R, G, and B). This increase in bit depth enables more brightness and color
levels to be captured for each sample in a scanned image. A 16-bit scan can sense
up to 65,536 discrete levels of brightness. This is considerably higher than the 256
levels of an 8-bit image.
This dramatic increase in sensitivity enables the scanner to capture subtle shadow
detail in the darker areas of an image. A 16-bit scan creates a 48-bit image. This of
course means 16 bits of data for the R, G, and B channels in an RGB scanner. The
result is tremendous clarity, but unfortunately a dramatically larger file size.
How Can a 48-Bit Scan Help Me?
If you are scanning negatives and need the highest quality possible, scan at 16-bit
depth. The resulting 48-bit scan will provide much more brightness values to work
with in your image editor. When you apply a Levels or Curves adjustment to the
resulting image, you have much more data to work with than with a traditional
24-bit image. Convert to 8-bit after you make your tonal corrections by choosing
Image, Mode, 8 Bits/Channel (see Figure 12.4).