The Effects of Optical Defocus on the Legibility of the Tumbling-E and Landolt-C

Abstract

BACKGROUND: The Tumbling-E and Landolt-C are both used to test visual acuity in young, illiterate, or non-English-speaking patients. In this study, we sought to determine how optical blur affects visual acuity thresholds measured with both of these optotypes. METHODS: Ten subjects were tested with 4-position Tumbling-E and Landolt-Cs using the method of constant stimuli. The subjects were tested wearing their habitual corrections and with +1.50 D spherical and cylindrical lenses to simulate myopic refractive errors in the optical blur test conditions. RESULTS: Visual acuity thresholds for the Tumbling-E were slightly better for all test conditions, while the largest threshold difference between the test letters was found for the simulated against-the-rule astigmatism. For both test letters, vertical orientations were more difficult to resolve than horizontal. Also, the slopes of the psychometric functions were steeper for the Tumbling-E than for the Landolt-C. CONCLUSIONS: The Tumbling-E can be considered comparable with the Landolt-C except for the simulated against-the-rule-astigmatism test condition. Although the ranges are quite large, the average psychometric functions of the Landolt-Cs are flatter, which gives rise to the possibility that more confidence can be placed in the visual acuity thresholds obtained with the Tumbling-E.