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A. A 24 bit colour image uses 8 bits to represent each of the three colours (RGB = Red, Green, Blue) (3 x 8 = 24), hence each colour can be represented by 256 possible values which when combined gives over 16 million possible colours. A 48 bit colour image uses 16 bits to represent each of the three colours (RGB) (3 x 16 = 48), so each colour can be represented by 65,536 possible values, which when combined gives billions of possible colours. Clearly there is a dramatic increase in the potential to capture subtle tonal variation in a 48 bit colour image. BUT inevitably the file size is increased (doubled), with concomitant increases in processing time, and not all software can handle 48 bit images, and even those that can (e.g. Photoshop) have some limitations in the tools that can be used. The biggest benefit of 48 bit images is when doing curves or levels adjustments in Photoshop; with a 24 bit image a toothcomb effect can often be seen, resulting in posterization effects. Some more info can be found here.