An apparatus for measuring positions of marks on a substrate, includes an illumination arrangement for supplying radiation with a predetermined illumination profile across a pupil of the apparatus, an objective lens for forming a spot of radiation on a mark using radiation supplied by said illumination arrangement, a radiation processing element for processing radiation that is diffracted by the mark, a first detection arrangement for detecting variations in an intensity of radiation output by the radiation processing element and for calculating therefrom a position of the mark, an optical arrangement, a second detection arrangement, wherein the optical arrangement serves to direct diffracted radiation to the second detection arrangement, and wherein the second detection arrangement is configured to detect size and/or position variations in the radiation and to calculate therefrom a defocus and/or local tilt of the mark.