Owing to their electric charge, electrons can be deflected by electric or magnetic fields. Modern electron microscopes are equipped with round magnetic lenses. The main function of a round magnetic lens is to cause rays at an angle to the optic axis to spiral around the optic axis, but not to focus. However, in a round 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.
Objective lens in EMs normally uses a round magnetic lens. The round magnetic lens cannot form a concave lens and thus, spherical aberration cannot be corrected by any combination of cylindrically symmetric round magnetic lenses.
In general, round lenses in conventional EMs suffer from spherical aberration as well as off-axial coma. To eliminate the azimuthal or anisotropic coma, the axial magnetic field must change its sign with a dual lens consisting of two spatially separated windings with opposite directions of their currents . The axial chromatic coefficient (Cc) of the coma-free lens is significantly larger (≥ 50%) than that of standard objective lenses. Therefore, in order to obtain sub-Ångstroem resolution it is necessary to greatly minimize the chromatic aberration in coma-free lenses.
Note that in EMs except for alignment deflectors, stigmators, and multipole aberration correctors, all the other conventional optical lenses are cylinder symmetric.
A EELS spectrometer consists of a couple of lenses. For instance, Enﬁna ER has 2 hexapoles and 5 electromagnetic round lenses, 7 dipoles for alignment, and 1 quadrupole and 1 additional hexapole for astigmatism correction.
 H. Rose, Optik 34 (1971) 285.