Magnetic electron lens

A magnetic electron lens uses a solenoid around the beam axis; iron pole pieces concentrate its magnetic field.^[ref] A strictly axial trajectory in a perfectly uniform axial magnetic field experiences no Lorentz force, since $\mathbf{v}\times \mathbf{B}=0$. Near the entrance, a radial field component turns an advancing Electron azimuthally; the resulting sideways velocity in the axial magnetic field also produces a radial Lorentz force. This couples image rotation and focusing. Real trajectories bend smoothly, whereas a thin lens diagram compresses that bending into one effective plane.