Effect of Optical Defocus on Detection and Recognition of Vanishing Optotype Letters in the Fovea and Periphery

Purpose.: Vanishing optotypes (VOs) are pseudo high-pass letters whose mean luminance matches the background so that they “vanish” when the recognition acuity threshold is reached in the fovea. We determined the effect of increasing blur on acuity for these optotypes and conventional letters, in both foveal and extrafoveal viewing.

Methods.: Detection and recognition thresholds were determined separately for each of the 26 letters of both a conventional and VO alphabet, both in the fovea and at 10° in the horizontal temporal retina, under varying degrees of positive dioptric blur.

Results.: In the fovea, detection and recognition thresholds were similar for individual VOs, increased steadily with blur, and separated somewhat at higher levels of defocus (3 diopters [D]). While the recognition thresholds for VOs changed on average by 0.28 logarithm of the minimum angle of resolution (logMAR)/D, those for conventional letters changed more rapidly by 0.35 logMAR/D. In the periphery, recognition thresholds were significantly higher than detection thresholds for the VOs at 0 D blur; both thresholds increased steadily thereafter, converging as blur increased. Peripheral recognition acuity displayed a loss of only 0.09 logMAR/D. In both the fovea and periphery, the interletter variation in recognition acuity was much lower for VOs than conventional letters (0.04 vs. 0.09 logMAR).

Conclusions.: Outside the fovea, high-pass VOs display significant differences in their detection and resolution thresholds up to +7 D blur, with a logMAR/D loss of a quarter that of the fovea. The lower interletter legibility differences indicate that VO letters may be better stimuli from which to design clinical letter charts.