A method of operating a Post Column Filter PCF in a Transmission Electron Microscope or a Scanning, Transmission Electron Microscope, the PCF equipped to operate in a first mode, the so-called EELS mode, in which an Electron Energy Loss Spectroscopy EELS spectrum is formed, said EELS spectrum showing an energy range, the PCF equipped to operate in a second mode, the so-called EFTEM mode, in which an Energy Filtered Transmission Electron Microscope EFTEM image is formed, and the PCF comprising the following optical planes: • an entrance plane, • an image plane where in EELS mode the EELS spectrum is formed and in EFTEM mode the EFTEM image is formed, • a slit plane between the entrance plane and the image plane where in EFTEM mode an energy dispersed focus is formed, the PCF comprising the following optical elements arranged round an optical axis: • an energy dispersive element between the entrance plane and the slit plane, the energy dispersive element dispersing an incoming beam of electrons in an energy dispersed beam, • one or more pre-slit quadrupoles (Q2, Q3) between the energy dispersive element and the slit plane, • a multitude of post-slit quadrupoles (Q4, Q5, Q6, Q7) between the slit plane and the image plane, a multitude of multipoles for correcting aberrations, and • an energy selective slit that in EFTEM mode is inserted in the slit plane, the optical elements between the entrance plane and the slit plane in EFTEM mode forming an energy dispersed focus in the slit plane; and the one or more pre-slit quadrupoles between the energy dispersive element and the slit plane in EFTEM mode enlarge the energy dispersion caused by the energy dispersive element the method characterized in that in EELS mode the one or more pre-slit quadrupoles between the energy dispersive element and the slit plane do not enlarge the energy dispersion caused by the energy dispersive element. • an entrance plane, • an image plane where in EELS mode the EELS spectrum is formed and in EFTEM mode the EFTEM image is formed, • a slit plane between the entrance plane and the image plane where in EFTEM mode an energy dispersed focus is formed, • an energy dispersive element between the entrance plane and the slit plane, the energy dispersive element dispersing an incoming beam of electrons in an energy dispersed beam, • one or more pre-slit quadrupoles (Q2, Q3) between the energy dispersive element and the slit plane, • a multitude of post-slit quadrupoles (Q4, Q5, Q6, Q7) between the slit plane and the image plane, a multitude of multipoles for correcting aberrations, and • an energy selective slit that in EFTEM mode is inserted in the slit plane, the optical elements between the entrance plane and the slit plane in EFTEM mode forming an energy dispersed focus in the slit plane; and the one or more pre-slit quadrupoles between the energy dispersive element and the slit plane in EFTEM mode enlarge the energy dispersion caused by the energy dispersive element the method characterized in that in EELS mode the one or more pre-slit quadrupoles between the energy dispersive element and the slit plane do not enlarge the energy dispersion caused by the energy dispersive element.