Spherical aberration and the depth of focus of the human eye

Abstract

Purpose: The spherical aberration of the human eye typically changes as a function of accommodation level. It is also common for simultaneous vision bifocal contact lenses to be designed with spherical aberration to alter the depth of focus of the eye. We have studied the effect of various levels of positive and negative spherical aberration upon the depth of focus of the eye.
Methods: The level of spherical aberration of the eye was altered by using rigid contact lenses. The rigid lenses were positioned close to the eye, but not in contact with the cornea. Elliptical surfaces were used on the lenses to generate controlled levels of spherical aberration including; -2.3, -1.2, 0.0, +1.2, and +1.8D of longitudinal spherical aberration for a 4mm pupil. Ten young subjects were tested following cycloplegia. Visual acuity was measured with the Freiburg computer based method, for object vergences from +/-2.00D in 0.5D steps.
Results: Spherical aberration caused an expected loss of acuity for 0D (infinite) object vergence. For the negative spherical aberration conditions there was an expected shift of peak acuity towards positive object vergence and vice versa for positive spherical aberration. By plotting visual acuity versus object vergence, the slope of the fitted data indicated the depth of focus of the eye. Spherical aberration increased the depth of focus of the eye in a manner dependent upon the sign and magnitude of the spherical aberration and the object vergence. We show that these results are consistent with theoretical predictions based upon geometrical optics.
Conclusions: The results indicate that the natural changes in spherical aberration of the eye during accommodation will increase the depth of focus in the direction of intermediate distances. The use of spherical aberration in bifocal contact lenses will cause significant visual acuity loss for modest improvements in depth of focus.
CR: None