Does a tilted retina cause astigmatism? The ocular imagery and the retinoscopic reflex resulting from a tilted retina

An astigmatic dial viewed by a tilted retina will have only one line in focus, simulating astigmatic blur. We compare and contrast this optical situation to actual astigmatism. Photographs were taken of an astigmatic dial blurred with a cylindrical lens, and also of the same astigmatic dial tilted, simulating tilted retinal imagery. A model eye for retinoscopy practice was provided with a retina tilted 30° and was retinoscoped repeatedly with and without added confounding cylinder by five skilled retinoscopists. Photographs of the astigmatically blurred and tilted astigmatic dials were similar but not identical, as expected. The model eye with tilted retina showed no astigmatism by retinoscopy in 14/15 measurements and 0.25 D of astigmatism in one measurement. When confounding cylinder was present, the retinoscopy measurement was always within 0.25 D of the added cylinder. The ocular imagery resulting from a tilted retina can simulate astigmatic blur, but this is actually due to a type of curvature of field. Only a minute area of the tilted retina is viewed during retinoscopy, so the tilt has essentially no influence on the refnoscopic reflex.