A lithographic apparatus includes an illumination system configured to provide a beam of radiation of radiation; a support configured to support a patterning device configured to impart a pattern to the beam of radiation; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam onto a target portion of the substrate, the projection system and/or illumination system including a focusing element; a plurality of stop discs each having an aperture therethrough different from the size and/or shape of the apertures of the other stop discs; and a mechanism configured to exchangeably place a selected one of the stop discs adjacent to the focusing element.