An inspection apparatus includes an inspection optical system configured to a direct an inspection beam onto a surface of a substrate, the inspection optical system having an objective, a focus measurement optical system configured to receive a focus measurement beam, redirected by the substrate, from the objective, the focus measurement optical system having a movable reflective element configured to receive the focus measurement beam, and a control system configured to cause movement of the reflective element with a direction component along a beam path of the focus measurement beam and configured to determine whether the substrate surface is in the focus of the objective based on the focus measurement beam.