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O. Yehezkel; Discrepancies between Eye Models and Clinical Testing in Recognizing Defocused Targets. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1118.
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The defocus test prevents prescription of over-refraction includes verifying a 6/12 vision with addition of 1.00D. However, simulating such vision conditions by any of the popular eye models (e.g. Liou-Brennan, Navarro) with a 3mm pupil typical emmetropia and a non-accommodated eye with 1.00D defocus, yields a minimal recognisable Snellen letter corresponding to 6/24 whereas the 6/12 letter is completely blurred and unrecognisable. This discrepancy and several possible solutions for it were investigated
Using various eye models with optical design software and optical bench testing, several explanations were evaluated:1. Periodical structures (grating acuity), checkerboard recognition or sinusoidal modulation.2. Visual system's neural processing.3. Blur adaptation.4. Residual uncorrected astigmatism.
1. For periodic structures the increase in the depth of focus (DOF) possibly results from spurious resolution or Talbot imaging and cannot explain increase in DOF for single letters.2. Neural processing including interaction between adjacent receptors may resolve more blurred details, especially with a-priori knowledge about characters type (letters, numbers, etc).3. Subjects may improve 0.1-0.3 logMAR after adaption over time to few diopters of defocus. However, unlike the investigated phenomenon, blur adaptation is not instantaneous4. The effect of additional average high order aberration with pupil of 3mm is minimal. However, we demonstrated that residual low order aberration could explain the greater DOF.
A discrepancy regarding the minimal recognizable defocused letter is presented and shown possibly to result from residual uncorrected astigmatic aberrations, which are not included as part of the existing eye models.
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