Abstract: The beams from a plurality of off-axis guns aimed through a common deflection volume, as used in shadow-mask color cathode-ray tubes, even if made by constant convergence fields to converge at a given distance in the absence of deflection, will diverge when a common deflection field is applied; and the distance from the deflection zone to different parts of the screen is generally not constant.Therefore the convergence field applied to each must vary as the beams are deflected to different screen areas. Prior art schemes to do this have employed functions of the form AY + BY.sup.2 + CX + DX.sup.2, where Y is the vertical deflection and X is the horizontal deflection. The present invention produces a function AY + BY.sup.2 + CX + DX.sup.2 + EXY, or alternatively this function plus FY.sub.+.sup.3 where Y.sub.+ is only the positive value of Y. The square and cube functions are produced by the use of multiplier circuits.