Start from the second-order paraxial lateral chromatic aberration of the cathode lens, that is, from the well-known Recknagel-Apцимович formula in the imaging electron optical system, this paper investigates the minimum diffusing circle formed by the electron beam, studies the determination of the position of the optimal imaging plane, investigates the contour of crossover formed by the electron beam emitted from the whole photocathode surface, as well as the diffusion circle on the cathode surface formed by the electron beam emitted from the origin, and depicts the envelope of the electron rays formed in the imaging section. The present paper will help the readers to understand the moving electron trajectory in the image tubes, as well as its beam convergence and divergence in the imaging section.