Contributions of optical and non-optical blur to variation in visual acuity

To determine the relative contributions of optical and non-optical sources of intrinsic blur to variations in visual acuity (VA) among normally sighted subjects.

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 (σint) and VA for the unblurred tumbling E (MAR0). 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 (σopt). An index of non-optical blur (η) was defined as 1 - σopt/σint. A control experiment was conducted on five subjects to evaluate the effect of stimulus duration on MAR0 and σint. RESULTS.: The logarithm of the minimum angle of resolution (logMAR₀) for the briefly presented E was correlated significantly with log σint (r = 0.95, p < 0.01), consistent with previous work. However, logMAR₀ was not correlated significantly with log σopt (r = 0.46, p = 0.11). For subjects with logMAR₀ equivalent to ~20/20 or better, logMAR₀ was independent of log η, whereas for subjects with larger logMAR₀ values, logMAR₀ was proportional to log η. The control experiment showed a statistically significant effect of stimulus duration on logMAR₀ (p < 0.01) but a non-significant effect on σint (p = 0.13).

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.