an imaging device , such as an eem , includes an electric / magnetic lens used to focus pulsed electrons emitted from an object on to a target plane . before a pulse of emitted electrons reaches the lens , electrons are spatially separated in dependence on their respective kinetic energies and are then subject to a time varying electric field that keeps the final focal plane constant for a wide variety of different energy electrons . the electric field compensates for variations in the image focal length caused by a spread in kinetic energies , causing the electrons to be focused proximate the target plane , reducing chromatic aberration . the varying electric field may be provided by varying an electric potential at the lens by , for example , varying a voltage supplied to an electrode at the lens . this potential effectively varies the focal strength of the lens in time , in order to compensate for variations in kinetic energies of electrons arriving at the lens , effectively keeping the image plane position constant .