Patent Number: 
Section: claims

1. A magnetic lens for generating a magnetic imaging field to focus charged particles emitted from a sample, the lens comprising a central pole piece and an outer pole piece disposed about the central pole piece, wherein the lens comprises at least one magnetic moveable element for movement relative to at least one of the pole pieces, whereby a focal length of the lens is variable by said movement of the at least one magnetic moveable element. 2. A magnetic lens as claimed in claim 1 wherein said movement of the at least one magnetic moveable element changes the magnetic circuit between the pole pieces. 3. A magnetic lens as claimed in claim 1 wherein the central pole piece comprises a magnetic moveable element which comprises a magnetic part which, in a first position, forms at least part of the central pole piece whereby a first magnetic circuit is generated and, in a second position, forms part of the outer pole piece whereby a second magnetic circuit is generated, different from the first magnetic circuit. 4. A magnetic lens as claimed in claim 1 wherein the at least one magnetic moveable element comprises a first magnetic part and a second magnetic part, said first magnetic part and second magnetic part being separated by a non-magnetic part. 5. A magnetic lens as claimed in claim 4 wherein the at least one magnetic moveable element is moveable between at least a first position and a second position, wherein, in the first position it defines a first focal length of the lens and, in the second position it defines a second focal length of the lens. 6. A magnetic lens as claimed in claim 4 wherein the first magnetic part comprises a magnetic elongate member. 7. A magnetic lens as claimed in claim 4 wherein the first magnetic part has a sample facing end which has an external diameter which decreases in the direction of the sample. 8. A magnetic lens as claimed in claim 4 wherein the second magnetic part comprises a magnetic annular member which is disposed about the first magnetic part. 9. A magnetic lens as claimed in claim 8 wherein the second magnetic part has an outer surface profiled for engagement with a complementarily profiled inner surface of the outer pole piece. 10. A magnetic lens as claimed in claim 4 wherein the second magnetic part has a maximum external diameter which is greater than the minimum internal diameter of the outer pole piece. 11. A magnetic lens as claimed in claim 1 wherein the central pole piece and the outer pole piece are symmetrical about an axis of the lens and the outer pole piece is annularly disposed about the central pole piece and co-axial therewith thereby defining an annular space therebetween, the lens further comprising an energizing means disposed in the annular space. 12. A magnetic lens as claimed in claim 1 wherein the central pole piece comprises a body, a moveable element being arranged for movement therein. 13. A magnetic lens as claimed in claim 12 wherein the body has a through-bore for accommodating therein the moveable element. 14. A magnetic lens as claimed in claim 12 wherein the moveable element is moveable between at least a first position and a second position with respect to the body of the central pole piece, wherein, in the first position, the moveable element is positioned with respect to the body whereby the first and second magnetic parts of the moveable element are in connection with the body such that the first and second magnetic parts form part of the central pole piece thereby defining a first focal length of the lens and, in the second position, the moveable element is positioned with respect to the body whereby the first magnetic part of the moveable element is in connection with the body such that the first magnetic part forms part of the central pole piece and the second magnetic part of the moveable element is in connection with the outer pole piece such that the second magnetic part forms part of the outer pole piece, thereby defining a second focal length of the lens. 15. A magnetic lens as claimed in claim 1 wherein the outer pole piece has a sample facing end which extends inward toward the central pole piece, wherein said end has an internal diameter which decreases in the direction of the sample. 16. A magnetic lens as claimed in claim 1 wherein, in the first position, the central pole piece does not extend out of the lens beyond the outer pole piece and, in the second position, the central pole piece extends out of the lens beyond the outer pole piece. 17. A magnetic lens as claimed in claim 1 wherein the outer pole piece comprises a magnetic moveable element which comprises a magnetic part which, in a first position, forms at least part of the outer pole piece whereby a first magnetic circuit is generated and, in a second position, forms part of the central pole piece whereby a second magnetic circuit is generated, different from the first magnetic circuit. 18. A magnetic lens as claimed in claim 1 wherein the lens provides at least two different magnifications. 19. A magnetic lens as claimed in claim 18 wherein the lens provides at least three different magnifications. 20. A method of focusing charged particles emitted from a sample comprising:providing a magnetic lens in proximity to the sample for generating a magnetic imaging field to focus the charged particles;energizing the magnetic lens to generate the magnetic imaging field;causing charged particles to be emitted from the sample; andfocusing at least some of the charged particles emitted from the sample to produce a charged particle image of the sample;wherein the magnetic lens comprises a central pole piece and an outer pole piece disposed about the central pole piece, wherein the lens further comprises at least one magnetic moveable element for movement relative to at least one of the pole pieces, whereby a focal length of the lens is variable by said movement of the at least one magnetic moveable element. 21. A method as claimed in claim 20 comprising energizing the lens when at least one moveable element of the lens is in a first position thereby defining a first focal length of the lens and providing the lens in proximity to the sample at a first working distance and energizing the lens when the moveable element is in a second position thereby defining a second focal length of the lens and providing the lens in proximity to the sample at a second working distance. 22. The method of claim 20, wherein the step of causing charged particles to be emitted from the sample includes irradiating a surface region of the sample with radiation thereby causing the emission of charged particles from the surface region and further comprising a step of analyzing the energy of the focused charged particles. 23. An analyzer for analyzing a selected area of the surface region of a sample comprising: a sample holder for holding the sample; a source of radiation for causing the emission of charged particles from a sample placed on the sample holder; a magnetic lens for focusing at least some of the charged particles emitted from the sample; and analyzing means for analyzing the energy of the focused charged particles emitted from the sample, wherein the magnetic lens comprises a central pole piece and an outer pole piece disposed about the central pole piece, wherein the lens comprises at least one magnetic moveable element for movement relative to at least one of the pole pieces, whereby a focal length of the lens is variable by said movement of the at least one magnetic moveable element. 24. The analyzer of claim 23, wherein the source of radiation includes a source of x-ray or ultraviolet radiation for causing the emission of photoelectrons from the sample, and wherein the analyzer means includes and electron analyzing means for analyzing the energy of the focused photoelectrons emitted from the sample.