Prescribing cylinders: the problem of distortion.

Astigmatic spectacle corrections produce distortion of retinal images. Under binocular conditions, small amounts of monocular distortion may significantly alter the sense of spatial localization, producing symptomatic binocular distortion. From examination of the nature and sources of monocular distortion, a rational basis may be developed for methods to minimize distortion when necessary. Adaptation to distortion occurs both by interpretive and physiological mechanisms, with the physiological mechanism appearing to be a type of rotational anomalous retinal correspondence. Through better knowledge of distortion and adaptive mechanisms, a revised set of practical guidelines for prescribing cylinders is presented.