Older observers' tolerance of optical blur: age differences in the identification of defocused text signs

To determine if visual aging affects the ability to identify blurred text signs in daytime or nighttime viewing conditions, the photopic and mesopic legibility thresholds of young and old adult observers were compared at three levels of optically induced acuity (Experiment 1). For both age groups, legibility was reduced by nighttime luminance and degraded acuity. Surprisingly, older observers were better than younger ones in identifying defocused (optically blurred) text in both daytime and nighttime conditions. In Experiment 2, older observers were also superior to younger ones in identifying defocused standard and spatially matched novel text signs. These findings indicate that older observers' tolerance of optical blur is a generic ability, rather than one explained by familiarity with the low-pass optical profile of specific signs. Consistent with the notion that factors beyond acuity contribute more importantly to text legibility for older than for younger observers, acuity was a stronger predictor of legibility thresholds in the younger group. Actual or potential applications of this research include the need to consider functional acuity in visual screening protocols and the design of text displays, as well as the utilization of compensatory letter-recognition training for older observers or others with diminished acuity.