The wavefronts used to generate blurred letters with fourth order aberrations (spherical aberration and secondary astigmatism) were used to calculate a large number (
n = 31) of optical and image quality metrics. Metric amplitude was then correlated with logMAR visual acuity observed for these same conditions (all of the data in
Figure 1A, C and D). The scattergrams in
Figure 2 show 3 examples of optical and image-quality metrics, which were reasonably well correlated with logMAR visual acuity (left panels) and 3 examples that were poorly correlated (right panels). In each case (good correlation and poor correlation), we illustrate examples of wavefront-based, PSF-based and OTF-based metrics. The metric of “pupil fraction” (PFSt, the fraction of pupil area for which the optical quality of the eye is reasonably good, Cheng, Thibos, & Bradley,
2003) was well correlated with acuity (
R = −0.837), whereas RMS wavefront error (RMSw) was poorly correlated (
R = 0.493). Also, two PSF metrics produced different correlations: the standard deviation of intensity values in the PSF, normalized to diffraction-limited value (STD,
R = −0.816) and PSF half-width-at-half height (HWHH,
R = 0.365). Finally, visual Strehl ratio (VSOTF, the contrast-sensitivity-weighted OTF divided by contrast-sensitivity-weighted OTF for diffraction limited optics, Cheng, Himebaugh, Kollbaum, Thibos, & Bradley,
2004) correlated well with logMAR (
R = −0.822), whereas the metric designed to capture the phase changes in the image, OTF/MTF ratio (VOTF), was poorly correlated with acuity (
R = −0.182). It is interesting to see that the metric of VOTF successfully divided the data points in to two distinct parts. The data points at the bottom area of
Figure 2F represent visual acuity obtained under the aberration conditions that introduced large phase shifts (VOTF < 1), whereas the data points at the top area of
Figure 2F represent visual acuity obtained under aberration conditions that maintained phase (VOTF ≈ 1). For those aberrations that did not introduce phase shifts (VOTF ≈ 1), visual acuity deteriorated due to decreased contrast, which had little effect on VOTF. Therefore it is not surprising to see many of the data points spread horizontally around the VOTF value of 1. Interestingly, we found that good visual acuity was also obtained from some aberration conditions with large phase shifts. Good acuities under these conditions are possible because these large phase shifts resulted in multiple ghost images (e.g. sample images in
Figure 1C and D) each of which could be resolved. Such fortuitous legibility is less likely to occur with multiple letter presentation and overlapping ghost images. Future studies designed specifically to examine the importance of spatial phase might clarify the utility of the metric VOTF for predicting the impact of phase shifts on vision.