The effect of astigmatic axis on visual acuity measured with different alphabets in Roman alphabet readers

Pedro M Serra, Michael J Cox, Catharine M Chisholm Bradford School of Optometry and Vision Sciences, University of Bradford, Bradford, UK Objective: Astigmatism produces meridional variations in the retinal blur pattern, thus interacting with object spatial detail and altering visual performance as the axis changes. This study investigates the influence of astigmatic axis orientation on visual acuity (VA) for four alphabets used worldwide. Methods: Visual acuity was measured monocularly in 25 Roman alphabet users (mean age: 25.6±7.5 years) using computer-presented logarithm of the minimum angle of resolution (logMAR) charts with letters from four different alphabets (Arabic, Chinese, Roman, and Tamil). VA was assessed under the effect of four optical conditions: best distance correction and three astigmatic conditions (using a +2.00 cylindrical diopter trial case lens with its axis oriented at 180, 45, or 90 degrees). For each alphabet, single optotypes were presented on a monitor viewed from a distance of 4.0 m, and a matching technique was used to identify the letters. Results: The degradation in VA with astigmatic defocus was influenced by the alphabet used (p