At first, the particle is considered as a highly aberrated spherical lens, so that its focal length is gained, and then the equivalent thin lens is found under the condition of the equal focal length and equal light distance.

• 首先将颗粒作为高象差的球形透镜求出其焦距,再由等焦距和等光程条件找出与球形透镜等价的薄透镜。