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.