Trilobites had to redevelop their corneal lenses after each moult, since the old lenses remained with the discarded exoskeleton. Earlier, Miller & Clarkson (1980) were able to reconstruct three main stages of the post-ecdysially developing lenses in the schizochroal compound eye of the Devonian trilobite Phacops rana milleri Stewart, 1927. In this present work it is shown that the conical, then saucer-like and later wave-shaped proximal profile of the lens in these developmental stages is consistent with a Huygensian correction for spherical aberration as postulated for the adult eyes of some other trilobites by Clarkson & Levi-Setti (1975). The focal length of the developing lens is determined as a function of the lens thickness comparing and fitting the theoretically calculated Huygensian profiles to the experimentally reconstructed real lens surfaces. Using an empirical quadratic function fitted onto the variation of focal length versus lens thickness, a probable series of change of form of the developing lens in Phacops rana milleri is reconstructed computationally. On the basis of the geometric optical model presented, a further possible transitional stage between the shape of the last stage of the post-ecdysially developing lens and its mature form can be derived. Using geometric optical formulae for thick lenses and paraxial approximation, many features of image construction have been estimated for the post-ecdysial development of the eye. The actual position of the retina below the lens, and whether this changed during post-ecdysial development, remains unknown from fossilized material. It has been possible, however, to calculate object position and magnification at all stages of post-ecdysial development, from the shape and thickness of the lens. Likewise, the positions of the retina for which Phacops rana milleri could take advantage of its spherically corrected Huygensian lenses are established here. It is probable that the retina was fixed or moved little during the post-ecdysial stages. If so the eye was myopic in the earliest developmental stage, but thereafter could see sharply at a distance of a few millimetres to a few centimetres from the visual surface. Depending on the receptor separation in the retina, the depth of focus estimated was several centimetres so that the depth in object space could reach one decimetre, over which the image was in focus in the developing eye.