Reliability of visual performance measurement under optical degradation.

Small groups of experienced subjects wore three types of bifocal contact lens which were shown to reduce visual performance at distance and near conditions. Monitor-based contrast sensitivity, Pelli-Robson contrast threshold, and high- and low-contrast visual acuity were each measured on two separate occasions. For each bifocal contact lens type the test-retest difference did not increase significantly with reduction in visual performance. Despite this the reliability of the visual performance measures was shown to be reduced compared to previous reports with normal well-corrected subjects and the test-retest repeatability coefficients (95% confidence limits) increased as the average visual performance declined. The poor reliability is of interest to the practitioner fitting bifocal contact lenses, and confirms previous suggestions that reliability may be related to visual degradation (e.g. ocular disease). In addition Pelli-Robson contrast thresholds were shown to be correlated with the spatial frequency represented by the presumed fundamental frequency of the component letters.