Concentric-design rigid bifocal lenses, part III: predicting vision from optical measurement

Abstract

While the main purpose of previous optical investigations of contact lenses has been to elucidate the effects of differences in design, there have been no previous reports of measurements of both optical performance and visual performance of the same contact lenses. In this study optical and visual results were compared and models developed to predict visual performance from the optical performance measurements of rigid concentric-design bifocal contact lenses. Optical performance was measured using an EROS solid state modulation transfer function (MTF) system with apertures of 2 to 6mm and contact lens decentration of 0 to 2mm. Visual performance was measured with a monitor-based contrast sensitivity system, Pelli Robson charts and high and low contrast visual acuity charts. Polymethylmethacrylate (PMMA) bifocal contact lenses were made on a back-surface design in both centre-distance and centre-near designs. Central optic zone diameters varied from 1.8 to 3.4mm, while the peripheral optic zone diameter was fixed at 75mm. Multiple regression analysis models were empirically derived to include the measured pupil diameter, decentration and central optic zone diameter. Best predictions (adjusted multiple correlation, R2: range 0.46–0.80) of visual performance were found with the MTF measurements made with a 4mm aperture. The average measured pupil diameter of the five presbyopic subjects was 2.8mm. These models indicated that the MTF was a useful measure of changes in lens design and that, given the correct conditions of measurement, good predictions of visual performance could be made. Hence, changes in the MTF of bifocal contact lenses are a useful indicator of changes in vision.