引力透镜公式计算所有的变量，比如距离和密度，对光何时离开背景星系以及行走了多远的距离给出更好答案。
The gravitational lens equations account for all the variables such as distance and density, and provide a better idea of when light left the background galaxy and how far it traveled.
用费马原理推导了透镜的物像公式.
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。
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.
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