An apparatus for investigating and / or modifying a sample with charged particles, in particular a scanning electron microscope, is provided. The apparatus comprises a beam (1, 2) of charged particles, a shielding element (10) having an opening (30) for the beam of charged particles to pass through, wherein the opening (30) is sufficiently small and the shielding element (10) sufficiently closely positioned to the surface (20) of the sample to reduce the influence of charge accumulation effects at the surface on the beam of charged particles.