1. An image processing apparatus for converting multi level image data into half tone image data comprising: a receiver for receiving items of multi level image data, said items of multi level image data associating positions in an image with a respective one of a range of shades; a bit mask store storing data representative of set of binary bit mask arrays, each of said arrays being associated with a respective shade of said range of shades, the entries in said arrays defining a pattern of dots and gaps for representing said associated shade; and a conversion unit operable to convert an item of multi level image data into half tone image data by: selecting the bit mask array associated with the shade for an item of multi level image data being converted; identifying one or more entries in said selected bit mask utilising the position identified by said item of multi level data; and outputting said one or more identified entries in said bit mask array as half tone data for said position, wherein the binary bit mask arrays associated with at least some pairs of consecutive shades in the mid range of said range of shades are such that the majority but not all of the pattern of dots defined by a bit mask array of said pair for a lighter shade is included as a subset of the pattern of dots defined by a bit mask for the darker shade of said pair.