The influence of a homogeneous axial magnetic field on the essential electron optical properties of lens raster systems is investigated with the aid of a digital computer. Whereas the electron optical transparency of the raster is reduced with increasing magnetic induction, its influence on the shape of the viewing-current characteristic is relatively small for the magnetic fields necessary for practical purposes. Owing to the focusing properties of the magnetic field, the current density profiles at the viewing screen are confined, and the resolution of the system is limited only by the raster hole spacing.