The term chromatic aberration is related to the energy of the electrons.
Electrons are not monochromatic. Electrons emerge from the gun at a whole range of energies and are bent by the objective lens to different degrees; electrons that have lost energy are bent more strongly. Thus, once again, electrons from a point on the specimen form a disk image, as for spherical aberration. The radius (rchr) of the disk is given by:
Where Cc is the chromatic aberration coefficient of the lens (length), ΔE is the energy loss of the e, Eo is the initial beam energy, and P is the semiangle of collection of the lens. While ΔE in the incident electron beam is < 1 eV. It is typically 15-25 eV for a good fraction of the electrons coming through thin foil 50–100 nm thick. Chromatic aberration gets worse for thicker foils as this leads to a higher fraction of inelastically scattered electrons which may be subject to such effects.