Effects of target distance and pupil size on letter contrast sensitivity with simultaneous vision bifocal contact lenses.

Using simulated presbyopia we measured the effects of target vergence and pupil size on letter contrast sensitivity while subjects wore accurately centered simultaneous vision bifocal contact lenses. These experiments examined the increase in depth of focus provided by two bifocal contact lenses, and the effects of pupil size on near and distance vision. Monocular contrast sensitivity was assessed with three different designs of soft contact lenses. First, as a single vision control, we measured contrast sensitivity with a Bausch & Lomb, Optima 38 lens corrected for distance. We then measured contrast sensitivity with an Allergan Optical, Hydron Echelon, which is a diffractive simultaneous bifocal and a CIBA Vision, Spectrum, which is a concentric design, center-near, two-zone simultaneous vision bifocal. In general both bifocal contact lenses improved contrast sensitivity at near but decreased contrast sensitivity at distance (when compared to the single vision lens). With a fixed 3.5-mm artificial pupil, the Echelon lens had two clear peaks of sensitivity, one at distance (0.00 D) and one at near (-2.00 D), but the Spectrum lens had no clear peaks. Performance of the Spectrum lens depended on its center zone size. Near (-2.00 D) and distance (0.00 D) contrast sensitivity was measured with pupil sizes that ranged from 1 to 6 mm. The results showed that visual performance with a diffractive lens was almost independent of pupil diameter up to 3 to 4 mm but, as predicted, performance with the two-zone lens depended critically on pupil size.