A lithographic apparatus includes an illumination system including a field faceted mirror including a plurality of field facets and configured to receive radiation from a radiation source and form a plurality of images of the radiation source on corresponding pupil facets of a pupil faceted mirror. Each of the field facets is configured to provide an illumination slit at a level of a patterning device. The illumination slits are summed together at the level of the patterning device to illuminate the patterning device. First blades are configured to block radiation from a radiation source and each first blade is selectively actuable to cover a portion of a selectable number of field facets. The field faceted mirror further comprises partial field facets, the partial field facets being configured to produce a partial illumination slit at the level of the patterning device, and the pupil faceted mirror further includes pupil facets corresponding to the partial field facets. The partial field facets are configured to produce an illumination slit that is summed with the summed illumination slits of the field facets and/or correct for non-uniformity in the summed illumination slits of the field facets. Second blades are selectively actuable to cover a portion of a selectable number of partial field facets.