1. A device for localization of a tissue variation in an examination subject for a biopsy, comprising: an x-ray imaging system; a control unit configured to operate the x-ray system to obtain an overview image of an examination subject in whom tissue variations exist, while the examination subject is located in a position in said imaging system; said imaging system comprising a display and being configured to display the overview image at said display, as a displayed image, with said tissue variations being visually perceptible in said displayed image; said control unit being configured to operate said x-ray imaging system to also obtain a first image of the examination subject in said position and a second image of the examination subject in said position; a computer configured to combine said first and second images to form a stereo image of the subject, said first and second images having respective overlapping regions in said stereo image; and said computer being configured to automatically determine dimensions of an x-ray field that is generated by said x-ray system that penetrates the examination subject and, from said dimensions, to determine a volume of the examination subject in which a biopsy can be implemented, with said subject in said position in said x-ray system, using said stereo image to show a tissue variation to be biopsied in said biopsy, among said tissue variations in said subject, and to superimpose a visually perceptible frame on said overview image in said displayed image at said display that represents dimensions of said x-ray field limited to said overlapping regions, in order to allow an immediate determination by a viewer of the displayed image as to whether said tissue variation to be biopsied is within said frame and thus can be biopsied while said patent remains in said position in said x-ray imaging system.