This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
The primary quality of electron optical elements (electromagnetic lenses) in electron microscopes (EMs) is usually degraded by secondary effects. They are aberrations, which can be categorized by three groups: geometric, chromatic, and parasitic aberrations, such as spherical aberration, diffraction aberration, astigmatism, coma aberration, chromatic aberration, and higher-order aberrations. All such defects will perturb the focusing properties of the corresponding lenses. In practice, electron lenses cannot be made free from aberrations. The most severe aberrations are the spherical aberrations and the chromatic aberrations. There are additional aberrations which should be considered at higher spatial resolutions, e.g. more widely categorized by coherent and incoherent aberrations.
Glaser’s famous bell-shaped model suggested that in order to attain the minimum focal length and the minimum aberrations of a magnetic lens used in a TEM the specimen should be put at the middle of the gap of a pair of symmetrical polepieces , e.g. called objective polepiece.
In a round magnetic lens, the focusing effect of the magnetic field increases too quickly with distance from the optical axis and the high angle rays are focused much more strongly. That is, there will always be significant aberrations if the lens is round.
Because the intensity of Ronchigram, formed at the Fraunhofer diffraction plane, varies significantly with angle, and this variation is a very sensitive function of lens aberrations and defocus , Ronchigram is a very useful way to characterize and optimize the electron probe in STEM mode.
 Cosslett, V. E. 1991. Fifty years of instrumental development of the electron microscope, in Advances in Optical and Electron Microscopy, Barer, R. and Cosslett, V. E. Eds., Academic Press, London, 215–267.
 J.M. Cowley, Electron diffraction phenomena observed with a high resolution STEM instrument, J. Electron. Microsc. Tech. 3 (1986) 25-44.