An electron microscope system and a method of measuring an aberration of the electron microscope system are disclosed. An aperture filters an electron beam at a diffraction plane of the electron microscope to pass through electrons having a selected energy and momentum. A displacement of an image of the passed electrons is measured at a detector in an image plane of the electron microscope. An aberration coefficient of the electron microscope is determined from the measured displacement and at least one of the energy and momentum of the passed electrons. The measured aberration can be used to alter a parameter of the electron microscope or an optical element of the electron microscope to thereby control the overall aberration of the electron microscope.