A scatterometer configured to derive a property of a substrate, includes an optical arrangement that produces a beam of radiation. An objective lens is arranged to focus the beam of radiation onto a target on the substrate. The optical arrangement is arranged to change the divergence of the beam incident on the objective lens, thereby changing spherical aberration caused by the objective lens on the beam focused on the target. A detection arrangement is arranged to detect the beam of radiation after reflection or scattering from the substrate.