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Visual acuity is perhaps the most well-known and most important measure of visual function. The concepts of “20/20” and “the big E” are familiar to patients and physicians in all fields of medicine. Visual acuity can be measured rapidly and inexpensively, with low-cost charts available commercially, online for printing, and increasingly for mobile devices.
Clinically, visual acuity is considered one of the vital signs of the eye and is measured at a predetermined distance, often using a Snellen chart. Developed in the 1860s, this chart has several design flaws, such as nongeometric progression of letter size and variable number of letters used per line (eg, “E” at the top of the chart and ≥5 letters on the 20/20 line). When measuring acuity in epidemiologic surveys or as an outcome in clinical research, the Snellen chart is insufficiently standardized, and other charts have been developed to address these deficiencies.1 The most well known of these is the retroilluminated logMAR (logarithm of the minimum angle of resolution) chart that was used in the Early Treatment Diabetic Retinopathy Study (ETDRS) and has since become the standard method of measuring visual acuity in prospective clinical research.2
Brady CJ, Eghrari AO, Labrique AB. Smartphone-Based Visual Acuity Measurement for Screening and Clinical Assessment. JAMA. 2015;314(24):2682-2683. doi:10.1001/jama.2015.15855