In STEM, there is only a condenser system, which is a lens used to form a fine probe. Sometimes this lens is also called objective lens. The correction of the spherical aberration (Cs) of the pre-field lens is much cheaper comparing to post-field corrections. The main advantage of such correction is to reduce the beam tails so that a fine beam can be positioned at a specified column of atoms and does not spread its intensity into neighboring columns significantly. This tail spilling is critical for high resolution Z-contrast (HAADF) imaging and EELS analysis.
For a well-aberration-corrected HAADF STEM, the distinguishability is much greater, e.g. one even was able to seperate the Mo and Sr atom columns (Mo1.7/Sr1.7=1.19) (see details at page4788).