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 imaging system to obtain an overview image of an examination subject containing tissue variations therein; said control unit being configured to operate said x-ray imaging system to also obtain a first image of the examination subject and a second image of the examination subject; a computer that combines said first and second images to form a stereo image of the subject, said first and second images having respective overlapping regions; and a computer that automatically determines dimensions of an x-ray field that penetrates the examination subject and that causes a visually perceptible frame to be displayed at a display connected to the computer that represents said dimensions of said x-ray field limited to said overlapping region