Can anyone explain what it means to have an RGB mode with 16 bits per channel? I understand that 8 bits can give 256 different values per channel, and with three channels, you get 256 x 256 x 256 = 16.7 million colours. But why would one need 16 bits per channel? 16 bits per channel means, to me, 65536 values per pixel. With 3 channels, this means 65536 x 65536 x 65536 or several trillion possible combinations per pixel, which is unthinkable.
Can anyone also explain why, when saving a photoshop document as a .bmp file, why do you get the options to save it as 16 bit or 32 bit? I thought that a .bmp could not be more than 24 bit (3 x 8 bits, or three bytes, one byte for each colour 0-255). In the 32 bit mode, what is the extra 8 bits for? I understand why you can save as 8 bit (grayscale or indexed colour) and 1 bit (line art). Also, 4 bit is confusing, What's that for?