Effects of spherical and astigmatic defocus on acuity and contrast sensitivity: a comparison of three clinical charts

Two contrast sensitivity charts (Vistech and Pelli-Robson) have become available to the eye care practitioner. Their value as clinical tools for the assessment of visual function may be enhanced because of either an insensitivity to the effects of optical focus or a hypersensitivity to defocus. We compared the sensitivity to defocus of these charts to the traditional Snellen chart by examining the effect of up to +/- 5.00 D of spherical and astigmatic defocus on performance with each chart. In order to simulate the two types of clinical examination scenarios, tests were performed both with and without mydriatic/cycloplegic agents. The Pelli-Robson contrast sensitivity chart was very resistant to the effects of all types of optical defocus. As predicted, the high spatial frequencies on the Vistech chart were sensitive to defocus. However, although contrast sensitivities for the low frequencies were affected less, optical defocus produced significant decreases in low frequency Vistech contrast sensitivity. In addition, the Vistech chart was very insensitive to axis 180 blurring lenses. There was no indication that either contrast sensitivity chart was more sensitive to defocus than the standard Snellen chart.