The invention provides 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 a 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 magnetic moveable element, thereby enabling a zoom facility for changing the magnification of an image. The movement of the moveable element preferably changes the magnetic circuit between the pole pieces. Also provided is a method of focusing charged particles emitted from a sample and a charged particle energy analyzer, such as an imaging photoelectron spectroscopy system.