1. A method for operating an automatic x-ray exposure unit having a radiation detector composed of a matrix of detector elements, comprising the steps of: selecting a predetermined dose which produces a complete exposure at said radiation detector of an examination subject; during a first time span, activating an x-ray source to emit a first dose pulse by selective adjustment of an x-ray source voltage for dimensioning said first dose pulse so as to be insufficient for generating a complete exposure even with a smallest density of said subject; during a second time span following said first time span, serially reading out image data from said radiation detector generated as a result of said first dose pulse and storing said image data in a first image memory; calculating a grayscale value frequency of occurrence distribution of the image data in said first image memory; and during a third time span following said second time span, generating a primary image of an examination subject dependent on said distribution and storing said primary image in a second image memory, and adding said image in said first image memory to said image in said second image memory to produce a final image.