In a cathode ray oscillograph, the focusing of beam on the screen is achieved by

In a cathode ray oscillograph (CRO), focusing of the electron beam is achieved using an electrostatic lens formed by applying different electric potentials to cylindrical electrodes, such as the focusing anode. This controls the beam diameter on the screen. While magnetic fields can also focus electron beams, convex lenses are not used, and the standard method in CROs is electrostatic focusing via electric potential.