Effects of Target Size and Test Distance on Stereoacuity.

Target size and test distance effects on stereoacuity were investigated in 24 subjects using a three-dimensional monitor. Examination 1: Target Size Effects. The test distance was 2.5 m for 0.1°, 0.2°, 0.5°, and 0.9° target sizes; crossed parallax was presented in 22-second units. Average stereoacuity values for 0.1°, 0.2°, 0.5°, and 0.9° target sizes were 59.58 ± 14.86, 47.66 ± 13.71, 41.25 ± 15.95, and 39.41 ± 15.52 seconds, respectively. Stereoacuity was significantly worse with a 0.1° target than with 0.2°, 0.5°, and 0.9° target sizes (P = 0.03, P < 0.0001, and P < 0.0001, resp.). Examination 2: Test Distance Effects. Test distances of 2.5, 5.0, and 7.5 m were investigated for a 0.5° target size; crossed parallax was presented in 22-second units. Average stereoacuity values at 2.5 m, 5.0 m, and 7.5 m test distances were 44.91 ± 16.16, 34.83 ± 10.84, and 24.75 ± 7.27 seconds, respectively. Stereoacuity at a 7.5 m distance was significantly better than at distances of 2.5 m and 5.0 m (P < 0.0001 and P = 0.02, resp.). Stereoacuity at a 5.0 m distance was significantly better than at 2.5 m (P = 0.04). Stereoacuity should be estimated by both parallax and other elements, including test distance and target size.