The present invention relates to an autofocus aperture stop (5, 6) in a triangulating autofocusing device (21) for a microscope (40), wherein the autofocus aperture stop (5, 6) comprises at least one diaphragm opening (3, 4) with which a measuring beam pencil (34) used for the autofocusing and running in the direction of the optical axis (18) of the autofocusing device (21) can be limited in its cross section, wherein the diaphragm opening (3, 4) of the autofocus aperture stop (5, 6) is arranged in a decentred position at a spacing from the optical axis (18) of the autofocusing device (21), wherein a decentred autofocus measuring beam (36) can be generated by the diaphragm opening (3, 4) in one half of the cross section (17) of the measuring beam pencil (34).