A radiation beam modification apparatus for controlling a property of a beam of radiation in a lithographic apparatus includes a flexible sheet provided with a plurality of apertures, and a positioning apparatus comprising a first rotatable member and a second rotatable member, wherein a first end portion of the flexible sheet is coupled to the first rotatable member, a second end portion of the flexible sheet is coupled to the second rotatable member and a central portion of the flexible sheet extends between the first rotatable member and the second rotatable member. The apertures may be used to control the numerical aperture of a projection system of a lithographic apparatus.