an electron gun for producing and directing at least one electron beam along a beam path comprises a beam forming section and a main lens section for focusing the electron beam . the main lens section includes first and second electrodes arranged along the beam path and each having an aperture through which the electron beam is passed , and an auxiliary electrode located between the first and second electrodes and having an aperture through which the electron beam is passed . the aperture of the auxiliary electrode is wider than those of the first and second electrodes . different voltages are applied to the first and second electrodes , and the auxiliary voltage between the voltages is applied to the auxiliary electrode . an electrostatic field formed between the first and second electrodes is corrected by placing a correcting electrode within the bathtub - shaped auxiliary electrode which is responsive to the auxiliary voltage such that the electron beams are more effectively converged .