Photolithographic apparatus (1) including an illumination unit (3). The illumination unit (3) comprises, in this order, a radiation source unit (11), a first optical system (5) and an optical waveguide (17). The apparatus (1) further includes a second optical system (7) and a mask table (9). The apparatus (1) is adapted in such a way that both the stepper mode and the scanner mode are selectable modi, and comprises means by which the illumination unit (11), when being operative, has a slit-shaped static illumination field with a width s which is variable between s.sub.min and s.sub.max while substantially maintaining energy within the static illumination field.