Low contrast charts effectively differentiate between types of blur.

Test charts with three different contrast levels were used to measure acuity of six subjects at distance and near. Reduction in measured acuity as a result of refractive defocus was shown to be independent of contrast level. Reduction in measured acuity with diffusive blur was greater on the medium and low contrast charts than on the high contrast chart. We advocate the use of charts with two different contrast levels to help differentiate between causes of visual degradation.