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L.N. Thibos, A. Bradley, R.A. Applegate; Determination of the Eye’s Far-point from an Aberration Map . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4192.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We sought a method for deducing the far-point of an aberrated eye from an ocular aberration map. Such a method is necessary for determining best refraction and for implementing wavefront-guided treatments. Methods: Accommodation was paralyzed and pupils dilated in both eyes of 100 subjects. Subjective refractions determined the spectacle correction needed to maximize visual acuity for high-contrast letters. Indicated sphero-cylindrical refractive errors were corrected with trial lenses when measuring monochromatic aberrations (633nm) with a Shack-Hartmann aberrometer. One definition of the far point is the center of curvature of that spherical wavefront which fits the aberrated wavefront best. Two different fitting algorithms were used: least-squares fitting and matching the meridionally-averaged paraxial curvature (i.e. spherical equivalent). Alternatively, the far point can be defined as that location in space where an object should be placed in order to maximize the quality of the retinal image. Through-focus calculations of the retinal image for each eye indicated that stimulus vergence needed to maximize image quality, defined by 24 different metrics. Results: The predicted far point is infinity (i.e. stimulus vergence K =0) since sphero-cylindrical refractive errors were corrected during aberrometry. Least-squares fitting of a spherical wavefront was a clear failure, predicting a mean K = -0.38D for the study population. The best method was a pupil-plane metric called pupil fraction, defined as the fraction of the pupil area for which wavefront error is small (i.e. RMS < wavelength/4). Distribution of K for this metric was symmetrically distributed about the predicted zero value (mean K =–0.006D, SD=0.29D). Spherical equivalent was also symmetrically distributed about zero, but with slightly higher variance (mean K = -0.0025 D, SD=0.36 D). The full description of results includes rank ordering of all 26 metrics used to establish the far point and the mean magnitude of error associated with each metric. Conclusions: Several successful methods for using an aberration map to locate the far point of the eye have been identified. One unsuccessful method is equivalent to locating the far point based solely on the Zernike coefficient for defocus.
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