A comparison of the Snellen chart and the S-chart for visual acuity assessment in amblyopia.

A standard Snellen chart and a chart that controls contour interaction effects and gives equal numbers of acuity trials for all letter sizes (the S-chart) were used to compare visual acuities for 30 amblyopic patients (17 with central fixation, 13 with eccentric fixation). No statistically significant difference appeared between S-chart and Snellen acuity in the acuity range 20/30 to 20/80. When amblyopic acuity was below 20/100, S-chart and Snellen acuities differed significantly.