An electron microscope includes an electron beam source, a first electromagnet, a second electromagnet and a detector. The field generated by the first electromagnet has an effect of three lenses subsequently arranged along the beam path. A first lens of these lenses is arranged upstream of the object plane and focuses the beam at the object plane. The second lens of these three lenses is arranged downstream of the object plane. The third lens of these three lenses generates an image of a diffraction plane of the second lens at the detector. The magnetic field generated by the second electromagnet has an effect of a fourth lens and can be changed in order to change a size of the image of the diffraction plane of the second lens on the detector.