Infants' acuity at twenty feet.

Measurement of duration of fixation was used to assess the ability of 6-, 12-, 24-, and 36-week-old infants to discriminate black-and-white square-wave gratings from a homogeneous gray field at a distance of 20 ft (6.1 m). Acuity was estimated as the smallest stripe width detectable by at least 18 of the 24 infants in a given age group. The strip width was considered detectable if it was fixated longer than a homogenous gray field. The results are consistent with estimates of infants' threshold acuity obtained at target distances less than 5 feet (1.5 m). Because of the 20-ft viewing distance, these infants' acuity in Snellen notation can be more confidently estimated.