For a 200 kV microscope, with partly corrected spherical aberrations ("to the third order") and a Cs value of 1 µm, a theoretical cut-off value might be 1/"q"max = 42 pm. The same microscope without a corrector would have Cs = 0.5 mm and thus a 200-pm cut-off. The spherical aberrations are suppressed to the third or fifth order in the "aberration-corrected" microscopes. Their resolution is however limited by electron source geometry and brightness and chromatic aberrations in the objective lens system.