Measuring contrast sensitivity with inappropriate optical correction

Spatial frequency-selective minima (notches) in the contrast sensitivity fu nction (CSF) because of defocus can mimic those that occur with ocular dise ase. We examined the influence of measurement conditions on CSF shape in si mulated clinical testing. CSF notches occurred with almost all levels of de focus for all subjects. Multiple notches were found under some conditions. Notches were found with defocus as small as 0.50 D. Effects of induced asti gmatism depended on the orientation of the target. Notches were apparent in defocus conditions after stimulus size and room illuminance were modified and when subjects had insufficient accommodation to compensate for hypermet ropic defocus. The equivalent of notches was not noted with the Pelli-Robso n chart. As defocus-induced CSF notches may be mistaken for functional loss , careful refractive correction should be conducted prior to clinical or ex perimental CSF measurement, even at low spatial frequencies. (C) 2000 The C ollege of Optometrists. Published by Elsevier Science Ltd. All rights reser ved.