The change of spherical aberration during accommodation and its effect on the accommodation response

Theoretical and ray-tracing calculations on an accommodative eye model based on published anatomical data, together with wave-front experimental results on 15 eyes, are computed to study the change of spherical aberration during accommodation and its influence on the accommodation response. The three methodologies show that primary spherical aberration should decrease during accommodation, while secondary spherical aberration should increase. The hyperbolic shape of the lens' surfaces is the main factor responsible for the change of those aberrations during accommodation. Assuming that the eye accommodated to optimize image quality by minimizing the RMS of the wave front, it is shown that primary spherical aberration decreases the accommodation response, while secondary spherical aberration slightly increases it. The total effect of the spherical aberration is a reduction of around 1/7 D per diopter of stimulus approximation, although that value depends on the pupil size and its reduction during accommodation. The apparent accommodation error (lead and lag), typically present in the accommodation/response curve, could then be explained as a consequence of the strategy used by the visual system, and the apparatus of measurement, to select the best image plane that can be affected by the change of the spherical aberration during accommodation.