Abstract: The focal length of the electrostatic lens in an oscilloscope tube can be calculated analytically when the potential distribution is known along the axis of the lens. Thus an empirical formula of the focal length is ob- tained from several simplifying models with the help of simulation calculation. Combined with the theory of normal principal point in electrostatic lens,a principle that there exist two sets of parameters that are equivalent in calculating can be derived from the empirical formula of focal length.

Key words: electrostatic lens, normal principal point, empirical formula of focal length, linear fitting