Reflections on retinoscopy

It has often been stated that static retinoscopy findings are not affected by the examiner's refractive correction. To what extent this statement remains true is the subject of this statement remains true is the subject of this paper. The basic principle of retinoscopy is the Foucault test. In this test, a knife edge placed on the principal axis of an optical system (S) intercepts a bundle of rays coming out of (S). Depending on the position of the knife edge, various distributions of light and shadow can be observed on the anterior surface of (S). In retinoscopy it is either the entrance pupil of the examiner or the peephole of the instrument that plays the role of the knife edge. Therefore, any change in conditions that bring about a change in the "knife-edge-pupil" position will result in errors in static retinoscopy. Nevertheless, calculations show that these errors do not become significant under usual clinical conditions of static retinoscopy.