A lithographic apparatus includes an illumination system configured to provide a beam of radiation and projection system configured to project the radiation beam onto a target portion of a substrate. At least one of the illumination system and the projection system includes a focusing element for reflecting or refracting the beam. A plurality of stop discs is provided, each having an aperture therethrough, together with a disc positioner configured to place one of the stop discs adjacent the focusing element to control the numerical aperture (NA) of the projection system or illumination system. The apparatus further includes a disc changer configured to select one of the stop discs and provide the selected stop disc to the disc positioner, the disc changer being external to the projection system or illumination system.