BACKGROUND A high reproducibility of visual acuity estimates is important when monitoring disease progression or treatment success. One factor that may affect the result of an acuity measurement is the duration of optotype presentation. For times below 1 s, previous studies have convincingly shown that acuity estimates increase with presentation duration. For durations above 1 s, the situation is less clear. METHODS We have reassessed this issue using the Freiburg Visual Acuity Test with normal subjects. Presentation durations of 0.1 s, 1 s, and 10 s were assessed. RESULTS Confirming previous findings, in all subjects acuity estimates in the 1-s condition were higher than those in the 0.1-s condition, on average nearly by a factor of 2, equivalent to 3 lines. However, in 12 out of 14 subjects, acuity estimates increased further with a presentation duration of 10 s, on average by 23% (P=0.002), or roughly 1 line. Test-retest variability improved by 49% (P=0.003). These findings can be explained by a simple statistical model of acuity fluctuations. Cognitive processing may also be a relevant factor. Interestingly, most observers subjectively felt that they could perceive the optotypes best in the 1-s condition. CONCLUSIONS The results highlight the importance of standardizing presentation durations when high reproducibility is required.