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Renfeng Xu, Arthur Bradley, Larry Thibos, Gildas Marin, Martha Hernandez; The impact of Spherical Aberration, Stiles Crawford Apodization and Spatial Frequency on wavefront-based and subjective refractions. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4549. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Subjective refraction is independent of pupil size even for eyes with spherical aberration (SA). We tested the popular explanation that Stiles-Crawford apodization (SCA) biases subjective refraction towards the pupil center by attenuating marginal rays.
Aberrometry data were used for objective wavefront refractions that indentify the target vergence required to maximize metrics of image quality for varying levels and signs of SA and apodization, and a range of spatial frequencies (SFs). Subjective refractions for a variety of test stimuli were measured with phase plates that controlled SA levels, and apodization filters that controlled the Stiles-Crawford effect.
In the presence of SA, high SF image quality and subjective appearance of best focus are both achieved with a near paraxial focus irrespective of the presence or absence of pupil apodization. Optimum low SF image quality is achieved by focusing more marginal pupil regions, and thus best focus for low SFs changes with SA levels and apodization. For letter charts used in clinical refraction, optimum retinal image quality is achieved with a sphere lens that focuses rays entering the eye 1- 1.5 mm from the pupil center.
Clinical subjective refractions are independent of pupil size because retinal image quality is maximized when rays near the pupil center are preferentially focused, independent of SCA. Removing high spatial frequencies from the visual stimulus makes subjective refractions more dependent on pupil size in the presence of SA.
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