The defocus of the electrons, that differ by an energy ΔE from those (E0) in focus, can be given by,
ΔE -- The energy range of the electrons,
Cc -- The chromatic aberration coefficient of the objective lens,
E0 -- The accelerating voltage of incident electrons.
Note that due to inelastic scattering, the ΔE of the electrons after they penetrate a TEM specimen is greater the ΔE before they pass the specimen.
The variation of the high voltage generated for electron microscopes (especially TEMs) should be kept as small as possible because it induces defocusing of EM images due to the chromatic aberrations.