Oh and one other thing if you have an object with some pure color (RGB 100%,100,!00% or 50%,0,0) and a white light (or a light whith the same purity as your colors). You would get banding because the interpolation would get pure colors and this would cause problems especially in pictures with 8 bits per channel.
Example you have a 50% gray ball, and white light (both pure white) and you use 8 bit per channel files. then you would get a grayscale gradient on the surface, but you would se banding because there are only 256 pure colors of white and sice you are using 50% you would get only 128 colors in your image, now this is a bit too litle for the eye.
To remedy this you should use a light (or surface color), that is not pure say 1 R, 0.99 G, 0.995 B now the image quality and colors are about the same but no banding occurs because the the renderer has more intermediate colors to play with.