To control convergence or divergence of the electron paths within one or more beams, usually by adjusting a voltage or current in a circuit that controls the electric or magnetic fields through which the beams pass, in order to obtain a desired image or a desired current density within the beam.

The center of an earthquake and the origin of its elastic waves within the earth.

A point in the plane which together with a line (directrix) defines a conic section.

To guide particles along a desired path in a particle accelerator by means of electric or magnetic fields.

1. The point or small region at which rays converge or from which they appear to diverge.

2. To move an optical lens toward or away from a screen or film to obtain the sharpest possible image of a desired object.