The Prescription Is a Place: How Vertex Distance Rewrites Strong Lenses, and the Power at Which It Starts to Matter

A lens's power is not a property of the lens alone: it is the power at a stated distance from the eye, and moving the lens changes what the eye receives. This paper computes the effective power change when spectacles sit at a different vertex distance from the one used at refraction, for powers from 4 to 14 D in both signs, using the standard vertex formula. The changes are negligible below about 6 D, reach a quarter dioptre for a 4 mm shift at about 7.2 D, and exceed half a dioptre at the highest powers for the shifts that ordinary frame fit produces - long lashes, deep-set eyes, a frame worn low. The dispensing rule is to record vertex distance at refraction for every prescription of 8 D or more, to compensate the ordered power when the chosen frame's vertex differs by 3 mm or more, and to check vertex before questioning a strong prescription that a wearer rejects. This study is simulated, not measured.