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Norberto Lopez-Gil, Vicente Fernandez-Sanchez, Larry N. Thibos, Arthur Bradley, Jesson Martin; What Is The Amplitude Of Accommodation Of An Aberrated Eye?. Invest. Ophthalmol. Vis. Sci. 2011;52(14):837.
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Amplitude of accommodation (AA) is the dioptric distance between the far and near point which, although uniquely defined in an aberration-free eye, can vary with definition of best focus in the presence of spherical aberration (SA) which changes during accommodation. We study how changes in ocular SA that occur during accommodation affect objective measures of AA.
Based on published anatomical data we perform a theoretical ray-tracing study of accommodation obtained when using only the central pupil optics (paraxial refraction, not affected by SA) or the whole pupil, (e.g. minimum wavefront variance or which is affected by SA). The results are compared with AA measures obtained objectively (by wavefront sensing) and subjectively (subjects were instructed maintain "clear focus" of a target whose optical vergence was adjusted using a custom made Badal optometer). 82 subjects (23.5±2.6 years) were tested in a dioptric range that exceeded their AA.
For a 4.5 mm pupil, theoretical modeling shows that the lens shape changes responsible for the change in sign of SA during accommodation produce a significant reduction in the AA defined by a minimum rms refraction compared to a paraxial refraction. On average, experimental data reveal a measured objective AA of 1.48 D less than subjective AA. This large apparent error in accommodation includes a combination of lower than ideal accommodation by the subject because of depth of focus (on the first pass) and a lower objective measure of accommodation by an aberrometer measuring the minimum rms refraction (on the second pass). Conversely, if accommodation was designed to maintain minimum rms focus, and the objective instrument measured paraxial focus, objective measures of accommodation could exceed the stimulus demand.
Because of the change of SA during accommodation, objective measures of AA could be significantly different from subjective measures if the instrument’s pupil weighting does not mirror that of the human accommodative control system.
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