The process or a method of making a beam of radiation or particles converge. In an electron-beam device, such as a cathode-ray tube, two principal methods of focusing the beam are used.

In electrostatic focusing two or more electrodes at different potentials are used to focus the electron beam. The electrostatic fields set up between the electrodes cause the beam to converge; the focusing effect is controlled by varying the potential of one of the electrodes, termed the focusing electrode. The electrodes are usually cylindrical, mounted coaxially with the electron tube, and are used in conjunction with deflection plates in instruments such as cathode-ray tubes.

In electromagnetic focusing the action of a magnetic field is used to make the electron beam converge. The field is produced by passing direct current through a focusing coil, the focusing effect being controlled by varying the current through the coil. The coil is usually one of short axial length that surrounds the tube and is coaxial with it. Electrons of different energies converge at different points along the beam axis. Thus if the electron beam is not monoenergetic, longitudinal spreading of the nominal convergence point results. Electromagnetic focusing is used in conjunction with deflection coils in instruments such as cathode-ray tubes.