An image system for processing image data in the form of grey-scale value signals and for generating an image that can be displayed on a presentation device, in particular for a medical diagnostic device, has a circuit to harmonize the image data. This circuit is fashioned for automatic contrast increase by a new adjustment of the grey-scale value signals, and thus of the grey-scale value dynamic range as well, with the C-value defining the center of the grey-scale value range, being retained after the harmonization.