Estimating visual acuity by character counting using the Snellen visual acuity chart

Purpose This study examines visual acuity estimation with character counting, which can be used in subjects with nonorganic visual loss. Methods The right eyes of 35 healthy subjects were fogged with plus lenses and tested with the Snellen visual acuity chart ( Reichert 11180). Visual acuity and counting level were assessed under various degrees of fogging, up to a maximal fogged acuity of 20/ 200. Counting level was defined as the smallest line that subjects could count the number of characters correctly. For each counting level, the visual acuity that 95% of subjects could see equal to or better than was determined. Results A counting level of 20/ 10 estimates ( ie 95% chance) a visual acuity equal to or better than 20/30. Counting levels 20/ 15, 20/ 20, and 20/ 25 estimate visual acuities of at least 20/ 50, 20/ 80, and 20/ 80, respectively. Counting levels 20/ 30 - 20/ 60 estimate a visual acuity of at least 20/ 200. Conclusions Character counting appears to be a useful technique of obtaining a rough estimate of visual acuity in subjects unable to be tested by standard methods, such as those with non-organic visual loss.