Soft contact lens-induced longitudinal spherical aberration and its effect on contrast sensitivity

Some investigators have suggested that the poor quality of vision which some spherical, single vision, soft lens-wearing patients report may be a result of spherical aberration induced in the ocular system when a soft lens is placed on the eye. In this study, the longitudinal spherical aberration of spherical soft lenses, both on and off the eye, was calculated using an aspheric corneal model and two-dimensional ray tracing program. Specifically designed front-surface aspheric, soft lenses were produced which demonstrated levels of in-air power variation similar to that calculated for similar-parameter spherically surface lenses. The effect of these lenses on the visual performance of nine subjects was assessed by measuring changes in contrast sensitivity and high contrast visual acuity through 3- and 6-mm artificial pupils. Significant losses of contrast sensitivity were recorded for the spherically aberrated lenses with the 6-mm pupil but not with the 3-mm pupil. High contrast acuity was not affected by any of the aberrated lenses with either the 3- or 6-mm pupils. Theoretical calculations and the contrast sensitivity results indicate that negatively powered lenses produce significantly less spherical aberration in situ than positively powered lenses. Because the majority of the prepresbyopic soft lens-wearing population have low to moderate amounts of myopia, it would appear that soft lens-induced spherical aberration is unlikely to be responsible for the reduction in visual performance which some patients report when corrected with single vision soft lenses.