A focussing lens for focussing a charged particle beam onto a specimen at a predetermined landing angle. The focussing lens comprises at least one first electrode having a first aperture to generate a focussing electric field for focussing the charged particle beam onto the specimen and a correcting electrode having a curved surface to compensate for landing angle dependent distortions of the focussing electric field caused by the specimen. With the curved surface of the correcting electrode, it is possible to improve the focussing of a charged particle beam at landing angles that differ from the perpendicular landing angle.