The Effects of the Relative Legibility of Optotypes on Corrected Visual Acuity

Abstract

Purpose: The Purpose of this study is to investigate if the relative legibility of optotypes affects the corrected visual acuity in visual acuity test. Methods: After measuring the relative legibility of 'Landolt ring target', 'arabic number target', 'alphabet target' by showing as a single-letter-target in 24 subjects without specific ocular diseases and ocular surgery experience, the relative legibility of 0.8, 1.0, 1.25 row of vision according to type of target in 7 types of chart were compared. After then we compared by measuring the corrected visual acuity according to type of target by using binocular MPMVA test (#7A) in 60 myopic subjects. Results: In 3 types of target the worst relative legibility target was 'Landolt ring target' with legible distance of 98.97±4.57 cm and the best relative legibility target was 'alphabet target' with legible distance of 108.42±3.46 cm. There was no difference of the relative legibility according to type of chart or visual acuity level in the row of vision if other conditions are the same. In 1.0 and 1.25 row of vision the difference of relative legibility according to type of target was shown the statistically significant difference between 'Landolt ring target' and 'alphabet target' as −0.07±0.06 (p=0.02) and −0.06±0.06 (p=0.04) respectively. In myopia the difference of corrected visual acuity according to type of target was statistically significant difference between 'Landolt ring target' and 'arabic number target' as −0.04±0.02 (p=0.02) and it was especially remarkable in the low myopia. Conclusions: Measuring visual acuity with different optotypes could cause the errors in best vision measurement value because there was difference of the relative legibility according to type of target even though visual acuity level is same in the row of vision.

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“…The suggested test was performed to recognize the Landolt ring shape with a gap in one direction in the ring. It was adopted by the International Academy of Ophthalmology (IAO) in 1909 as a unit time based on minimum separable acuity [24]. Shape evaluation with Landolt ring has also been used in other fields.…”