1. An X-ray examination apparatus comprising: an X-ray source for forming an X-ray image, an X-ray detector for receiving an X-ray image, and an exposure control system for adjustment of the X-ray examination apparatus, wherein the exposure control system is arrange to group pixels of the received X-ray image in one or more clusters on the basis of each of their brightness values and on the basis of brightness values of the pixels surrounding each pixel, and to adjust the X-ray apparatus on the basis of the numbers of the clusters or the sizes of the individual clusters.