An illumination system of a microlithographic projection exposure apparatus includes first and second optical raster plates. An irradiance distribution of projection light on the first and second optical raster plates determines an angular light distribution of the projection light exclusively at a first portion and a second portion, respectively, of an illuminated field. The second portion is distinct from and arranged adjacent to the first portion. It is possible to produce different illumination settings in different adjacent portions on the mask. First and second Fourier optics establish a Fourier relationship between the first and second optical raster plates one the one hand and the first and second portion on the other hand. The first and second Fourier optics have a first and second focal length, respectively, that are variable in response to a focal length change command signal from a control unit.