1. A method for determining the position and/or radius of the limbus and/or the position and/or radius of the pupil of a patient eye, the method comprising the steps of: obtaining an image of the patient eye; providing a plurality of different ring-shaped comparison objects having respective radii and respective centers; correlating said image with said plurality of said comparison objects by determining a two-dimensional brightness distribution of respective filter responses of said comparison objects to yield an at least local maximum between said image and said comparison objects when there is a coincidence of one of said ring-shaped comparison objects and a ring-shaped jump in brightness in said image having the same radius and the same center; determining said comparison objects having a local best match with said image; and, selecting the position of the center of the comparison object having a local best match with said image as the position of the center of the limbus and/or the position of the center of the pupil.