Patent ID: 8362443

Claim:
An objective lens, comprising: a magnetic lens having an optical axis, the magnetic lens comprising an inner pole piece and an outer pole piece, wherein a gap is formed between a lower end of the inner pole piece and an inner end of the outer pole piece, wherein the outer pole piece has a substantially conical shape having a diameter D a at its inner end, and the inner pole piece has a diameter D i at its lower end, wherein the lower end of the inner pole piece is arranged at a distance H, when measured along a direction of the optical axis, apart from the inner end of the outer pole piece, wherein the inner end of the outer pole piece is a functional part of the magnetic lens disposed closest to an object plane of the objective lens, wherein the inner pole piece and the outer pole piece are displaceable relative to each other, and wherein the following conditions are fulfilled: 1.5 ·D i ≦D a ≦3· D i and 25°≦α≦70° where α=arc tan((D a −D i )/(2*H)).