Simulated in situ optical performance of bifocal contact lenses.

A model of the presbyopic eye in various viewing conditions is derived as a combination of average clinical data with a monochromatic eye model. The modulation transfer function (MTF), obtained through Fourier optics calculations, is used to define new metrics, which help to predict the visual performance of multifocal contact lenses. The model is applied to the optimization of concentric bifocal contact lenses (BCLs): the optimal center optimal zone diameter (COZD) is determined as a function of the lens addition and decentration, in order to achieve similar average performance in distance and near vision. Calculations predict a decrease of the overall visual performance of optimized bifocal lenses with increasing addition. They also show that a lens shift relative to the pupil has opposite effects on the performance of center near (CN) and center distance (CD) designs. The comparison between these two types of bifocals suggests that better overall performance is obtained when the near correction is supported by the center optical zone. These theoretical predictions are found to be compatible with anterior clinical results obtained with BCLs.