An improvement is made in a color picture tube having an inline electron gun for generating and directing a plurality of electron beams along coplanar paths toward a screen of the tube. The gun includes a main focus lens for focusing the electron beams which is formed by two spaced electrodes. Each electrode includes a portion having a plurality of apertures therein equal to the number of electron beams. Each electrode also includes a peripheral rim. The peripheral rims of the two electrodes face each other and the apertured portion of each electrode is within a recess set back from the rim. The improvement comprises the depths of the apertures in the electrodes being equal to the thickness of material of the electrodes. This aperture structure permits greater penetration of the electrostatic lines of the main focus lens into the electrodes behind the apertured portions of the electrodes. This penetration of electrostatic lines provides a converging field for off-axis electron beams.