Figure 4316 shows the quadrupole design for spherical aberration (Cs) correction. The optical axis, along which the electrons travel, is into the page. indicates the direction of the electron beam. The blue lines represent the magnetic field lines, while the red line represents the Lorentz force on the electrons. Figure 4316 also shows the quadrupole provides a simple focusing effect in one azimuthal direction while provides simultaneous defocusing in the perpendicular direction. Therefore, the quadrupole can be used to focus the beam into a line (called line focus).
Figure 4316. The quadrupole design for spherical aberration (Cs) correction.
Combination of four magnetic quadrupoles with at least three octupoles  is normally employed for spherical aberration correction at high accelerating voltages in STEM system due to its off-axis aberrations.
 Krivanek, O. L., Dellby, N., and Lupin, A. R. 1999. Towards sub-Å electron beams, Ultramicroscopy, 78,