Optimal spherical focus in the peripheral retina

It is well known that, in most eyes, astigmatism increases with the field angle. A simple reduced-eye model is used to demonstrate that, for point imagery in the peripheral retina, the combination of oblique astigmatism with elliptical entrance and exit pupils means that the retinal image with the optimal rotational symmetry is not necessarily at a focus corresponding to a 'best-sphere' correction. Equations are derived for the position of focus at which a circularly symmetric blur patch is obtained and for the dimensions of the patch in this and other image planes. Ray tracing through a wide-angle schematic eye is used to explore the validity of the simple model. It is shown that although the latter gives good predictions of retinal imagery for very small entrance pupils, it becomes less valid for larger, more realistic pupil diameters, due to the increasing importance of the effects of higher-order aberrations. Nevertheless, the simple model can still yield useful insights into through-focus, peripheral optical imagery.