An objective lens for focussing charged particles includes a magnetic lens and an electrostatic lens whose components are displaceable relative to each other. The bore of the outer pole piece of the magnetic lens exhibits a diameter Da which is larger than a diameter Di of the bore of the inner pole piece of the magnetic lens. The following relationship is satisfied: 1.5·Di≦Da≦3·Di. 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.