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 to yield a local best match 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; 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; said ring-shaped comparison objects being closed ring filters or part ring filters; and, a filter response (A) of said ring filters being computed for said image of the patient eye as a measure for the correlation of said image of the patient eye and said ring filters; wherein a local maximum filter response (A) is determined with respect to said ring filters; and, the position of the center and the radius of the ring filter, on which said filter response (A) is based, is selected as the position of the center of the limbus of the patient eye and as the radius of the limbus; and/or, as the position of the center of the pupil of the patient eye and the radius of the pupil.