Reliability of the Snellen chart

Historically, visual function has been assessed by determining the finest spatial detail that the visual system can discriminate. A letter acuity chart, such as the Snellen chart, is commonly used. This type of test is simple to perform and is sensitive to the most common sources of visual impairment, such as uncorrected refractive error, cataract, macular disease, and amblyopia. A recent article in the BMJ identified some of the factors reducing the Snellen chart's reliability, such as failure to test visual acuity at the right distance and under recommended levels of illumination.1 But other determinants inherent in the design of the Snellen chart also warrant consideration. During the measurement of visual acuity only the angular subtense of the letters should change as the subject reads down the chart, which is not the case with the Snellen chart. Variation in the number of letters on each line presents the subject with a task of increasing difficulty rather than providing an equivalent task at all …