Contributions of Optical and Non-Optical Blur to Variation in Visual Acuity

Purpose. To determine the relative contributions of optical and non-optical sources of intrinsic blur to variations in visual acuity (VA) among normally sighted subjects. Methods. Best-corrected VA of 16 normally sighted subjects was measured using briefly presented (59 ms) tumbling E optotypes that were either unblurred or blurred through convolution with Gaussian functions of different widths. A standard model of intrinsic blur was used to estimate each subject's equivalent intrinsic blur (sigma(int)) and VA for the unblurred tumbling E (MAR(0)). For 14 subjects, a radially averaged optical point spread function due to higher-order aberrations was derived by Shack-Hartmann aberrometry and fit with a Gaussian function. The standard deviation of the best-fit Gaussian function defined optical blur (sigma(opt)). An index of non-optical blur (eta) was defined as 1 - sigma(opt)/sigma(int). A control experiment was conducted on five subjects to evaluate the effect of stimulus duration on MAR(0) and sigma(int). Results. The logarithm of the minimum angle of resolution (logMAR(0)) for the briefly presented E was correlated significantly with log sigma(int) (r = 0.95, p < 0.01), consistent with previous work. However, logMAR(0) was not correlated significantly with log sigma(opt) (r = 0.46, p = 0.11). For subjects with logMAR(0) equivalent to similar to 20/20 or better, logMAR(0) was independent of log eta, whereas for subjects with larger logMAR(0) values, logMAR(0) was proportional to log eta. The control experiment showed a statistically significant effect of stimulus duration on logMAR(0) (p < 0.01) but a non-significant effect on sigma(int) (p = 0.13). Conclusions. The relative contributions of optical and non-optical blur to VA varied among the subjects and were related to the subject's VA. Evaluating optical and non-optical blur may be useful for predicting changes in VA following procedures that improve the optics of the eye in patients with both optical and non-optical sources of VA loss. (Optom Vis Sci 2011; 88:716-723)