Bit depth refers to the number of bits of data used to record each pixel in the image. The greater the bit-depth, the greater the range of tones and colours recorded, and the smoother the tonal transitions in your image. Most images are 8-bit, or may be referred to as 24-bit. (There are three colour channels in each image – red, green and blue – each containing 8-bit data, hence ‘24-bit’). Normally, this is plenty, and images don’t show any obvious steps in tone or colour. However, after heavy manipulation, blotchiness, banding or posterisation effects can occur. It is possible to save images in 16-bit mode, which produces far smoother tones, but for this you need a camera which can shoot RAW images (most digital SLRs) and a program that can edit them (like Photoshop).