Abstract By assuming the weak lens focal length expression to be the first term in a power series, a simple and accurate formula can be derived for the objective focal length of a magnetic electron lens. This extended weak lens formula remains accurate even at strong excitations. This technique has also been applied to some of the other optical parameters. Where this approach was not practical, purely empirical formulae were derived. The constants in the for mulae were determined using data published by previous authors. The accuracies of the formulae are sufficient for most engineering design purposes.