Method for operating a medical X-ray device when performing an X-ray examination, with the method steps: a) recording a plurality of first X-ray images of a body region in a first phase of the X-ray examination, wherein the plurality of first X-ray images is averaged according to a recurrence formula to generate a mask image , wherein the mask image is updated with each newly accrued first X-ray image , b) providing a first subsequent image and recording a second X-ray image of the body region in a second phase of the X-ray examination, wherein the second X-ray image represents the body region at a later recording time than the first subsequent image ; wherein when initially executing the steps b) to e) as the first subsequent image , the last updated mask image of the first phase is specified, c) determining a degree of deviation (D) relating to a deviation between the first subsequent image and the second X-ray image , d) determining an averaging amount (α) in dependence on the degree of deviation (D) according to a predetermined rule, wherein the determination of the averaging amount (α) is performed using the predetermined rule by means of a distribution function , which is a function of the degree of deviation (D), wherein the distribution function is provided in dependence on an operating parameter of the X-ray device , e) generating a second subsequent image from the first subsequent image together with the second X-ray image , wherein the averaging amount (α) specifies the proportions in which the first subsequent image and the second X-ray image are mixed thereby, f) forming an overall image from the mask image and the second subsequent image . a) recording a plurality of first X-ray images of a body region in a first phase of the X-ray examination, wherein the plurality of first X-ray images is averaged according to a recurrence formula to generate a mask image , wherein the mask image is updated with each newly accrued first X-ray image , b) providing a first subsequent image and recording a second X-ray image of the body region in a second phase of the X-ray examination, wherein the second X-ray image represents the body region at a later recording time than the first subsequent image ; wherein when initially executing the steps b) to e) as the first subsequent image , the last updated mask image of the first phase is specified, c) determining a degree of deviation (D) relating to a deviation between the first subsequent image and the second X-ray image , d) determining an averaging amount (α) in dependence on the degree of deviation (D) according to a predetermined rule, wherein the determination of the averaging amount (α) is performed using the predetermined rule by means of a distribution function , which is a function of the degree of deviation (D), wherein the distribution function is provided in dependence on an operating parameter of the X-ray device , e) generating a second subsequent image from the first subsequent image together with the second X-ray image , wherein the averaging amount (α) specifies the proportions in which the first subsequent image and the second X-ray image are mixed thereby, f) forming an overall image from the mask image and the second subsequent image . <EOS>