dal manuale di ps 7...

Bit depth--also called pixel depth or color depth--measures how much color information is
available to display or print each pixel in an image. Greater bit depth (more bits of information per
pixel) means more available colors and more accurate color representation in the digital image.
For example, a pixel with a bit depth of 1 has two possible values: black and white. A pixel with
a bit depth of 8 has 28, or 256, possible values. And a pixel with a bit depth of 24 has 224, or
roughly 16 million, possible values. Common values for bit depth range from 1 to 64 bits per
pixel.

In most cases, Lab, RGB, grayscale, and CMYK images contain 8 bits of data per color
channel. This translates to a 24-bit Lab bit depth (8 bits x 3 channels); a 24-bit RGB bit depth (8
bits x 3 channels); an 8-bit grayscale bit depth (8 bits x 1 channel); and a 32-bit CMYK bit
depth (8 bits x 4 channels). Photoshop can also read and import Lab, RGB, CMYK, and
grayscale images that contain 16 bits of data per color channel.


per il web conviene usare immagini a 8 bit mentre quando si lavora su immagini su predilige usare 16 ...