Seidel aberration theory shows that the spherical aberration of a spherically surfaced intra-ocular lens cannot be made zero but only minimized and that the optimum shape is independent of the aberration of the rest of the eye (i.e. the cornea). The optimum shape of an intra-ocular lens correcting for spherical aberration is almost plano-convex with the more curved surface facing the cornea. The resulting total Seidel spherical aberration of the whole eye cannot be made zero unless the cornea has an asphericity much higher than the population mean.