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(2), 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. (Optom Vis Sci 2011; 88: E798-E805)