Increasing bit depth can allow a scanner to capture color at more than eight bits per channel and transform it to eight bits for output to a file or image application program. Together with good dynamic range, this increased bit depth processing also can improve ability to obtain detail from the darker areas of an image.
Bit depth determines the precision of a scanner's tonal output.
With eight bits per channel, the CCD analog voltage signal from of an image pixel can be converted to only one of 256 values, ranging from 0 for the darkest to 255 for the lightest, as illustrated by upper line of input values in the graph on the left. This provides four times as many digital input values from the CCDs values as are available with a 24-bit scanner. A 36-bit scanner would offer even more precision with a range of 4096 input values per channel. A scan bit depth greater than that provided in output can help to obtain better detail in the shadow areas when some tonal shifts are made after the analog to digital conversion. Such tonal shifts are made to compensate for visual nonlinearity and are typically introduced via gamma correction, adjustment of the scan histogram midpoint, and from some other adjustments in the scanner driver software.