1. Apparatus for adjusting the contrast of an image formed of pixels in which each pixel has an initial brightness level that is within a range of brightness levels between a minimum brightness level and a maximum brightness level, the apparatus comprising: a counter constructed and arranged to count, for at least some of the pixels in the image and for each of the brightness levels within said range, the number of said pixels that have the same initial brightness level; a classifier constructed and arranged to divide said pixels into at least two types by identifying the or each set of pixels having initial brightness levels in which each of a plurality of adjacent ones of said brightness levels has more pixels than a threshold number, the pixels of said plurality of adjacent ones of said brightness levels being a first type, the remaining pixels being a second type; and, a contrast enhancer constructed and arranged to perform contrast enhancement separately on the or each set of pixels of the first type, wherein the threshold number is the average of the number of pixels per brightness level obtained in the counting step multiplied by a constant greater than 0 and less than or equal to 1.