An objective lens (100) for focussing charged particles comprises a magnetic lens and an electrostatic lens whose components are displaceable relative to each other. The bore of the outer pole piece (2) of the magnetic lens exhibits a diameter Da which is larger than a diameter Di of the bore of the inner pole piece (1) of the magnetic lens, wherein the following relationship is satisfied: 1.5-Di = Da = 3-Di, and wherein the lower end of the inner pole piece is disposed in a distance of at least 2 mm offset from the inner end of the outer pole piece in a direction of the optical axis OA.