An optical system (110) includes a lens unit (112) with a plurality of lenses. An out-of-focus point spread function of the lens unit (112) includes an annular intensity distribution with at least one ring-shaped side peak at a radial distance to a center point. A birefringent device (115) in an optical path of the optical system (110) is adapted to selectively attenuate the ring-shaped side peak in the out-of-focus point spread function of the lens unit (112).