The disclosure relates to measuring a target. In one arrangement, a measurement apparatus is provided that has an optical system configured to illuminate a target with radiation and direct reflected radiation from the target to a sensor. A programmable spatial light modulator in a pupil plane of the optical system is programmed to redirect light in each of a plurality of pupil plane zones in such a way as to form a corresponding plurality of images at different locations on the sensor. Each image is formed by radiation passing through a different respective one of the pupil plane zones.