Introduction

The eye can be expected to suffer from all the aberrations that we find in other optical systems, but with one essential difference. Man-made systems are usually designed with some symmetry. For example, most visual optical systems have rotational symmetry. In contrast, the eye is not rotationally symmetric, which is mostly due to uneven growth patterns in the different components. The lack of symmetry means that there is a degree of irregularity in the conventional aberrations. These can be readily observed on axis by slightly defocussing the eye while viewing bright point light sources. The irregular star shaped image is due to irregular aberrations, because regular aberrations would produce a uniform circular or elliptical (if astigmatism is present) defocus blur disc.

The significance of optical aberrations, whether regular or irregular, is not at all clear. The eye generally uses only foveal viewing for the discrimination of fine detail and thus it could be argued that the eye only requires good optical image quality over the region of the fovea, which has an angular subtense of about 2°. Since the fovea is about 5° off-axis, one would expect off-axis aberrations, such as coma and transverse chromatic aberration, to be present at the fovea. The other off-axis aberrations (astigmatism, field curvature and distortion) would also be present but perhaps not in significant amounts. This is because, as shown in Table 33.1, these aberrations vary as the square of the field angle or cubic for distortion and therefore build up slowly with field angle for small field angles.