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B. Vasudevan, IV, A. Bradley, L. N. Thibos, J. Martin, J. Nam, N. L. Himebaugh; Impact of Positive and Negative Defocus on Visual Acuity. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3955.
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Because of the profound differences in the retinal images generated by positive and defocused in an aberrated eye, we examined the visual impact of both types of defocus.
Twenty subject eyes participated in the study. All the subjects were cyclopleged with 1% cyclopentolate in right eye only, and aberrations were measured using COAS at 630nm. Forced choice staircase methods were used to measure letter acuity after subjective refraction. Defocus was controlled by inserting ophthalmic lenses in front of a 3 or 6 mm artificial pupil that was imaged into the eye’s pupil plane using a unit magnification telescope. Single letters and 3X3 clusters of letters were presented for 1 second, subjects used a keyboard to identify the (central) letter.
In most eyes, positive defocus produced a fairly linear increase in logMAR (mean slope = 0.5 logMAR/Diopter), but significant non-linear effects contributed to shallower slopes (mean slope = -0.32 logMAR/Diopter) with negative defocus when testing with single letters. Between -1 and -2 diopters, many subjects showed a dramatic decline in the effect of increased negative defocus, and in some cases single letter VA actually improved with increasing defocus. Within this dioptric range, many observers report performing the acuity task by simply reading one of the doubled images observed. Image doubling is consistent with computed images for these eyes based upon measured higher order aberrations and negative defocus. The different impact of positive and negative defocus on acuity was greatly reduced when using the 3X3 letter clusters. With this stimulus, the doubled target images were spatially superimposed with doubled images from the surrounding characters.
Our results indicate that, in eyes with positive spherical aberration, negative defocus can generate multiple or double images. This doubling was particularly detrimental to VA when using clusters of characters such as found in text. The loss of visual performance associated with positive and negative defocus may, therefore, be attributed to quite different types of image degradation, and thus the vision loss experienced by myopes and presbyopes/hyperopes is likely to be qualitatively different.
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