Apparatus for focusing a charged particle beam onto a surface, including a charged particle beam generator which is adapted to project the charged particle beam onto a location on the surface, thereby causing charges to be emitted from the location. The apparatus further includes an imaging detector which is adapted to receive the charges so as to form an image of the location, and an aberrating element which is positioned before the imaging detector and which is adapted to produce an aberration in the image. A processor is adapted to receive the image and to adjust at least one of the charged particle beam generator and a position of the surface in response to the aberration.