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The effects of optical defocus on visual acuity and contrast sensitivity

1 May 2005


Background: Recent studies using the Small Letter Contrast Test (SLCT) suggest that small letter contrast sensitivity (CS) is more sensitive than visual acuity (VA) to optical defocus. With the advent of the new versions of the SLCT, we determined the effect of optical defocus on VA and CS and established normative values for these new charts.

Methods: High and low contrast VA and CS (20/40 and 20/50 letter size) were assessed in 25 subjects under decreasing amounts of spherical blur beginning at + 1.50 D to plano DS.

Results: At all levels of defocus 2:0.5 D, 20150 CS proved more sensitive than high and low contrast VA and 20/40 CS. At defocus of 0.38 D, 20/50 CS was more sensitive than high contrast VA and 20/40 CS, but not significantly different from low contrast VA and 20/40 SLCT.

Conclusions: CS with 20/50 letter size is a sensitive approach to quantify visual resolution in the contrast domain. Sensitivity to blur equals or exceeds that obtained with standard tests.


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