It is well known that the normal human eye suffers from wavefront errors (WFEs) not correctable with spectacle lenses (Helmholtz,
1896; B. Howland & Howland,
1976; H. C. Howland & Howland,
1977; Smirnov,
1961; Walsh, Charman, & Howland,
1984). For highly aberrated eyes, the higher-order (HO) WFEs are of such a magnitude that, in many instances, spectacle corrections do not provide acceptable vision. Even with rigid gas-permeable (RGP) contact lenses, the residual aberrations of highly aberrated keratoconic eyes are well outside the normal range (Kosaki et al.,
2007; Marsack, Parker, Pesudovs, Donnelly, & Applegate,
2007; Negishi, Kumanomido, Utsumi, & Tsubota,
2007), leading to the investigation of wavefront-guided corrections designed to target patient-specific levels of HO WFE (Marsack, Parker, & Applegate,
2008; Sabesan et al.,
2007). Intuitively, one may think the larger the amount of residual RMS WFE, the poorer the visual performance. However, RMS WFE is not necessarily a good predictor of visual performance as measured by visual acuity (VA) because (a) individual aberrations do not impact visual performance equally (Applegate, Ballentine, Gross, Sarver, & Sarver,
2003; Applegate, Sarver, & Khemsara,
2002; Chen, Singer, Guirao, Porter, & Williams,
2005) and (b) individual aberrations interact to increase or decrease visual performance (Applegate, Marsack, Ramos, & Sarver,
2003; McLellan, Prieto, Marcos, & Burns,
2006). Several other metrics for quantification of optical quality have been proposed (Thibos, Hong, Bradley, & Applegate,
2004) and will be examined here with respect to VA.