The present invention relates to a multi-beamlet multi-column particle-optical system comprising a plurality of columns which are disposed in an array for simultaneously exposing a substrate, each column having an optical axis and comprising: a beamlet generating arrangement comprising at least one multi-aperture plate for generating a pattern of multiple beamlets of charged particles, and an electrostatic lens arrangement comprising at least one electrode element; the at least one electrode element having an aperture defined by an inner peripheral edge facing the optical axis, the aperture having a center and a predetermined shape in a plane orthogonal to the optical axis; wherein in at least one of the plurality of columns, the predetermined shape of the aperture is a non-circular shape with at least one of a protrusion and an indentation from an ideal circle about the center of the aperture.