There are some key issues about scanners and printers that need to be understood by anyone expecting to become proficient with high-quality digital imaging and printing. These issues or factors are: color depth, dynamic range, and resolution.
Color depth is just another way to say bit depth for scanners, and the same principles apply. A normal color original (print or film) will require a minimum 24-bit scan (8 bits of information per RGB channel; all scanners scan in RGB) to reproduce with adequate fidelity. This is the old “millions” scanner setting. Most modern scanners are 12-bit scanners. of colors simply overkill? Maybe, if you consider that until recently with the introduction of Adobe Photoshop CS, almost no one could edit or work with these high-bit files, and consequently, they were always down-sampled or converted by the scanner into 24-bit images. However, the advantages of adding more information up-front (more raw material or “headroom” to work with) in the scanning stage has become more obvious to people, especially those working with film versus those scanning prints where the density range is much lower .
Keep in mind that many high-bit scanners still output the files in 24-bit (some do true 48-bit output). Why only 24-bit output? Because humans can not really see 48 bits, and also because computer monitors operate in 24-bit mode (although most digital printers can accept 48-bit data, which is usually converted on the fly to 24-bit). However, you still want to be scanning in high-bit mode because a 36-bit scanner has more steps in the range of densities between deep shadows and light highlights than does a 24-bit scanner. (Literally, there are no more 24-bit scanners; they're either 36-, 42- or 48-bit.) A 24-bit scan means 8 bits per RGB channel, which equates to 256 possible brightness levels per channel 0-255, where 0 is pure black and 255 is pure white). A 36-bit color scan means 12 bits per channel or 4,096 possible values. That's a lot more tonal possibilities. And this is especially important in the dark shadows of positive or reversal film (which with negatives turn into highlights) where you want as many steps to differentiate unique detail as possible. Shadow detail is frequently what makes or breaks an image. The downside-and there's always a downside-to scanning and then editing high-bit image files is that increasing the bit depth increases the file size arithmetically. However, as scanning expert David Coons says, “Double the file size is a small price to pay for the 256-times increase in luminance accuracy you realize by moving to 16-bits per channel.”