Caustic figures combined with the second and third order astigmatisms, which frequently appear due to any constructional inaccuracies and misalignments in the electron lens system, are theoretically investigated by means of geometrical optics. The caustic lines in different caustic planes observed by the experiment are compared with the theoretical ones. It results that the spherical aberration constant and the combined astigmatic aberration constant can, respectively, be estimated by measuring the defocusing deviated from the focus. Then, the available information of the compensation of astigmatism by the “Stigmator” can be obtained from the caustic figures.