In an x-ray diagnostics installation for mammography, an automatic setting of the optimum beam quality ensues in combination with transparency matching. A set of exposure parameters corresponding to different thicknesses of the examination subject is stored in a memory (reference values). At the beginning of an x-ray exposure, a comparator compares the transparency signal (actual value) to the exposure parameters (reference value) interrogated from the memory dependent on the current thickness of the examination subject, and influences the x-ray tube voltage for the purpose of a rated/actual value comparison without interrupting the exposure.