This is an option that allows the User to automatically vary one or more applied voltages to obtain the best focus position.
The 'focus' is defined as the set of crossing points of the rays as they cross a test plane (which is at a position chosen by the User).
The program will adjust the electrode voltages iteratively to minimise the size of this focus, using the simplex technique.
The requirements of Users vary greatly, and so several different types of focus are available, such as single or multiple focus points.
Several different forms of restraint are also available, such as limits to the voltages or the angles.
The focus iterations can be combined with space-charge iterations.
This is a very powerful option, of great use for many problems.