Visual acuity with astigmatic blur

Purpose: To determine how high- and low-contrast visual acuities are affected by blur caused by crossed cylinder lenses.

Methods: Crossed cylinder lenses of power 0 (no added lens), +0.12 diopter sphere (DS)/-0.25 diopter cylinder (DC), +0.25 DS/-0.50 DC and +0.37/-0.75 DC were placed over the correcting lenses of the right eyes of eight subjects. Negative cylinder axes used were 15 to 180° in 15° step for the two higher crossed cylinders and 30 to 180° in 30° steps for the lowest crossed cylinder. Targets were single lines of letters based on the Bailey-Lovie chart. Successively smaller lines were read until the subject could not read any of the letters correctly. Two contrasts were used: high (100%) and low (10%). The screen luminance of 100 cd/m, in combination with the room lighting, gave pupil sizes of 4.5 to 6 mm.

Results: High-contrast visual acuities were better than low-contrast visual acuities by 0.1 to 0.2 log unit (1 to 2 chart lines) for the no added lens condition. Based on comparing the average of visual acuities for the 0.75 D crossed cylinder with the best visual acuity for a given contrast and subject, the rates of change of visual acuity per unit blur strength were similar for high contrast (0.34 ± 0.05 logarithm of the minutes of arc resolution/D) and low contrast (0.37 ± 0.09 logarithm of minutes of arc of resolution/D). There were considerable asymmetry effects, with the average loss in visual acuity across the two contrasts and the 0.50 D/0.75 D crossed cylinders doubling between the 165° and 60° negative cylinder axes. The loss of visual acuity with 0.75 D crossed cylinders was approximately twice times that occurring for defocus of the same blur strength.

Conclusions: Small levels of crossed cylinder blur (≤0.75 D) produces losses in visual acuity that are dependent on the cylinder axis. Crossed cylinders of 0.75 D produce losses in visual acuity that are twice those produced by defocus of the same blur strength.