A lithographic apparatus comprises an illumination system, a support constructed to support a patterning device, and a projection system. In pixel grid imaging, a large number of small optical spots are imaged onto the substrate surface using a micro-lens array (MLA). The z position of the MLA is adjustable in order to focus the spots on the substrate surface and/or to compensate for differences in height of the substrate surface. The focusing adjustment is based on an output of an ultrasonic distance sensor provided in the vicinity of the substrate surface.