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Xu Cheng, Arthur Bradley, Larry Thibos; Predicting the effects of ocular aberrations on visual performance. Journal of Vision 2004;4(11):1. doi: https://doi.org/10.1167/4.11.1.
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Foveal resolution is almost always limited by the eye's optical aberrations. Previous studies of the effect of the lower-order aberrations (defocus and astigmatism) on visual performance have shown that letter acuity is proportional to the magnitude of defocus. This is understandable because, to first approximation, the angular diameter of the blur circle on the retina for a point source of light is the product of pupil diameter and magnitude of defocus. Thus, for constant pupil diameter, as defocus increases the blur circle grows in size and therefore the dimensions of the smallest resolvable letter must also grow in size proportionally (Smith, et al., 1989, Optom Vis Sci, 66: 430–435). However, this explanation lacks generality because it ignores the effects of higher-order aberrations on image quality, fails to embrace the true shape of the point-spread function, and provides no insight into the case of the well-focused eye. Accordingly, the purpose of our study was to characterize the visual effects of various combinations of higher-order and lower-order monochromatic aberrations on visual acuity for monochromatic stimuli. We then attempted to account for these experimental results by quantifying the optical quality of the eye with a variety of scalar metrics (Thibos et al., 2004 J. Vision, 4:329–351). We found that several image-plane metrics designed to quantify the compactness of the eye's point-spread function were good predictors of visual acuity under aberrated conditions, especially those metrics that give less emphasis to the low intensity tails of the point-spread function (e.g. the visually-weighted Strehl ratio, R=0.8). Similarly, the pupil-plane metrics of wavefront quality that were most predictive were those that give less emphasis to wavefront errors near the pupil margin (e.g. fraction of pupil area for which wavefront quality is relatively good, R=0.76).
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