The art of retinoscopy is based theoretically and practically on the maintenance of a known distance between the eye of the patient and that of the retinoscopist. The length actually chosen is based on habit and convenience, and may be, and frequently is, modified to suit unusual circumstances. The dioptric effect of this distance is, of course, well known: Compensation is made (for infinity) by subtracting from the retinoscopic findings the number of diopters which is the reciprocal of the distance in meters. The observer places before the eye of the patient a lens which will cause the emergent light to come to a focus at some convenient and accurately measured distance. The reciprocal of this focal length (in meters) yields the dioptric value of the lens which causes parallel light to be focused at that specific distance; and this dioptric value subtracted from the lens actually in place yields
WEINTRAUB JD. RETINOSCOPY AT A DEFINITE DISTANCE. Arch Ophthalmol. 1935;14(3):458–463. doi:10.1001/archopht.1935.00840090144010
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