The invention describes a particle-optical apparatus arranged to focus a beam (1) of electrically charged particles with the aid of two particle-optical lens systems (10, 20). The lens action is achieved by magnetic fields, which fields are generated by permanent-magnetic materials (13, 23). In contrast to magnetic lenses equipped with a coil, it is not easy in the case of lenses equipped with permanent-magnetic material to alter the focusing magnetic field with the aim of altering the optical power. In an apparatus according to the invention, the optical power of the lens systems is altered by altering the energy with which the beam (1) traverses the lens systems (10, 20). This can easily happen by altering the voltage of electrical power supplies (14, 24).