Multi-beam lithography apparatus is used for writing patterns on a substrate 14 such as a wafer for ICs. The patterns may have details of various dimensions. In order to enhance the production rate, it is attractive to write fine details with a small spot 16 and large details with a large spot. It is known to vary the spot size by varying the emissive surface of the electron source. In accordance with the invention the spot size is varied by varying the size 22 of the beam limiting aperture 20, thus enabling optimization of the beam current in dependence on the spot size. A preferred embodiment is provided with an additional (condenser) lens 24 such that the object distance remains constant when the magnification of the lens system 18, 24 is varied.