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When describing the human eye, celebrated scientist Hermann von Helmholtz once wrote1 ‘Now it is not too much to say that if an optician wanted to sell me an instrument which had all these defects, I should think myself quite justified in blaming his carelessness in the strongest terms, and giving him back his instrument’.
Helmholtz's assertion about the optics of the eye was quantified about a century later by Smirnov2 in Russia and the Howland brothers in the US, who found that normal human eyes exhibit significant levels of high-order aberrations, which can be much higher in eyes with corneal pathology, for instance, keratoconus.3 Laboratory studies and modern clinical aberrometers have allowed a large number of eyes to be measured in the last decade showing that aberrations of the unaccommodated eye are dominated by low-order (spherocylindrical refractive errors), third-order (coma and trefoil) and fourth-order spherical aberration4 (SA), as well as chromatic aberrations.5
From the perspective of an optical design, therefore, Helmholtz was correct, since the average human eye with pupil diameters larger than 5 mm has more than 10 times the monochromatic aberrations of typical optical instruments, which normally fulfil Marèchal's criterion (root mean square (RMS)<λ/14). Moreover, in polychromatic light, the quality of the retinal image can be even worse due to the significant amount of chromatic aberration presented in every eye, >2D between both extremes of the visible expectrum.6
Although all human eyes have significant monochromatic aberrations, the population mean for many of these aberrations is approximately zero.7 The notable exception is SA and, therefore, a correction for the population average SA could improve retinal image quality for most eyes. However, the positive SA observed in unaccommodated eyes, becomes significantly negative when accommodating,8 and thus, a SA correction that improved image …