In Electrostatic Focusing, Electrostatic lens consists of three anodes, with the middle anode at a lower potential than the other two electrodes. An electrostatic focusing system is shown in figure.
In figure, two anodes and its electrostatic lines and equipotential surfaces are shown.
Potential difference is kept between these two electrodes so that an electric field is generated between them. Spreading of electric field is caused because of repulsion between electric lines.
If equipotential lines are drawn, as shown in figure, they would bulge at the center of the two anodes, as we know that electrons move in a direction opposite to that of electric field lines and equal-potential surfaces are perpendicular to the electric field lines so force on the electron is exerted in the direction normal to the equipotential surface.