The resolution of a charged particle beam, such as a focused ion beam (FIB), is optimized by providing an energy filter in the ion beam stream. The energy filter permits ions having a desired energy range to pass while dispersing and filtering out any ions outside the desired energy range. By reducing the energy spread of the ion beam, the chromatic aberration of the ion beam is reduced. Consequently, the current density of the ion beam is increased. The energy filter may be, e.g., a Wien type filter that is optimized as an energy filter as opposed to a mass filter. For example, to achieve useful dispersion the energy filter may use a quadrupole structure between two magnetic pole pieces thereby producing a combined quadrupole electric field and dipole electric field within a magnetic field.