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.

• 用假设弱透镜焦距表达式是一个幂级数中的第一项的方法,可以导出相当准确且便于实用的电磁电子透镜物镜焦距公式。