Owing to their electric charge, electrons can be deflected by electric or magnetic
fields. Modern electron microscopes are equipped with round magnetic lenses. The
magnetic flux is typically of the order of 1 T (Tesla), concentrated in a narrow circular gap
between two pole pieces. The magnetic lens in EMs requires not only a high magnetic field but also a narrow (highly confined) field distribution.
Figure 4951 shows the electron trajectories incident parallel to the axis for increasing values of lens strength (ω = (1+k2)1/2).
Figure 4951. The electron trajectories incident parallel to the axis for increasing
values of lens strength (ω). Adapted from 
Note that objective lens is the strongest lens in EMs and has the largest effect. In TEM systems, the first condenser and final projector lenses have short focal length and their designs are similar to that of the objective lens. However, the second condenser and the other projector lenses may be much weaker.
The magnification in TEM can be adjusted in a wide range (e.g. from x50 to x20, 000, 000) by changing the strength of the magnetic field in the magnetic lens and thus changing the focal length. Note that the image in TEM rotates with the change of magnification because the total magnetic flux strength of the lenses changes. Recently, image rotation-free TEMs have been developed by keeping the total flux strength constant even if the magnification is changed.
 W. Glaser: Grundlagen der Elektronenoptik (Springer, Wien, 1952).