Longitudinal spherical aberration of modern ophthalmic lenses and its effect on visual acuity

The longitudinal spherical aberration of those soft and hard contact lenses that have spherical surfaces is substantially larger than that of spectacle lenses. By applying at least one ellipsoidal surface, this type of aberration can be greatly reduced, and good conformity of the posterior surface to the cornea can be simultaneously attained. Theoretical and experimental studies were made of the effect of spherical aberration of ophthalmic lenses on visual acuity. The visual acuity measurements were made with specially designed spectacle lenses using four-position Landolt C's as test targets. The probabilistic nature of the visual resolution was considered in evaluating the results. It was found that excessive spherical aberration of ophthalmic lenses reduces visual acuity, particularly at low levels of illuminance.