Concentric-design rigid bifocal lenses,part II: Visual performance

Abstract

The pupil-size dependence of concentric-design simultaneous-vision bifocal contact lenses presents difficulties to thepractitioner as some compromise must be made. If unsatisfactory, this will result in vision unacceptable to the wearer. While concentric-design bifocal contact lenses have been used for at least 40 years, there have been few investigations of the visual or optical performances of this form of correction. This study, the second of two parts, examined the effects of lens design, central optic zone diameter (COZD), pupil size and contact lens decentration on the visual performance of rigid concentric-design bifocal contact lenses. Part I of this study examined the optical performance of the same bifocal contact lenses. Ten experimental, rigid, bifocal contact lens designs with varying COZD and of both Centre-Near (CN) and Centre-Distance (CD) format were investigated. The visual performance offive trained presbyopic subjects was assessed using monitor-based contrast sensitivity, Pelli Robson contrast thresholds at 4m, and high- and low-contrast visual acuity. Visual performance was affected by COZD, contact lens decentration, and pupil size. Repeatability of the visual performancemeasures was worse than in previous reports and correlated with the relative reduction in visual performance. The low-contrast letter chart was the test most sensitive to differences in bifocal contact lens design. Pupil coverage by the COZ needed to achieve optimal visual performance (equal distance and near vision) varied with visual test, lens decentration and pupil size. CN lenses might be preferred slightly as near vision would be enhanced by convergence-related pupil constriction. This also suggests that a slightly smaller COZD should be used with a CD lens than with a CN design. In general, there was good agreement between the visual performance and optical performance, reported in Part I 14 indicating that the Modulation Transfer Function (MTF) is a useful tool for examining the effects of changes in bifocal contact lens design.