A method is described that includes illuminating a patterning device pattern with a radiation beam having a symmetric illumination mode, the patterning device pattern comprising a first pattern feature that substantially diffracts radiation of the radiation beam, and a second pattern feature that does not substantially diffract radiation of the radiation beam, introducing an asymmetry, relative to an optical axis, in the substantially diffracted radiation using a phase modulation element, illuminating a radiation beam receiving element with radiation emanating from the phase modulation element to form a receiving element pattern that is related to the patterning device pattern, the receiving element pattern having first and second receiving element pattern features related to the first and second pattern features respectively, and determining information at least indicative of a focal property from positional information regarding the relative positions of the first and second receiving element pattern features.