By assuming the weak lens focallength expression to be the first term in a power series a simple and accurate formula can be derived for the objectivefocallength of a magnetic electron lens.
If the objective lens is further away from the compact disk than the focallength of the object lens then the cylindrical lens again creates an elliptical image on the photodetector array.
如果物镜与光盘之间的距离长于物镜的焦距，柱面透镜也会使在光电探测器阵列上的成像变成椭圆影像。
3
If the objective lens is closer to the compact disk than the focallength of the object lens then the cylindrical lens creates an elliptical image on the photodetector array.