For vision correction, our goal is to achieve perfect functional vision. In other words, we want to obtain the best visual outcome after vision correction. The questions are then, what kinds of ocular aberrations affect the visual performance the most, or which Zernike aberration may cause a particular visual symptom?
The technological advancement of aberrometry has given the vision correction industry the ability of precisely measuring the ocular aberrations in living human eyes. For the researchers and developers of vision correction techniques, it is very desirable to find some correlations between ocular aberrations and visual performance, at least optically.
Many authors[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] have studied different optical metrics to best describe the relationship between the ocular aberrations and the visual performance. Among the various potential metrics, some are based on the optical path difference, or the RMS wavefront error on the pupil plane, and some are based on the calculation of the point spread functions on the retinal plane. Classically, the RMS wavefront error has been widely used as a very useful optical metric when it is very small, as given by the MarÃ©chal approximation.  For normal human eyes,[12, 13] or even eyes corrected with adaptive optical systems,  the residual RMS wavefront error is still too large to apply the MarÃ©chal approximation. In fact, it is generally accepted that the pupil plane metrics do not correlate as well with visual outcomes as the retinal plane metrics. Even so, both sets of optical metrics are useful in understanding the characteristics of the ocular aberrations.
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