Vertex Distance Calculator

A lens gives a different effective power depending on how far it sits in front of the eye, which is why a strong spectacle prescription is not the same number as the contact lens that corrects the same eye. Enter the sphere, cylinder and axis with the vertex distance the prescription was written at and the one you want it at, and this returns the compensated power rounded to the quarter dioptre lenses are actually made in. Each principal meridian goes through the formula separately, so the cylinder changes by a different amount from the sphere and the axis is carried through untouched. Below about four dioptres the correction comes to less than one lens step, and the page says so rather than pretending otherwise.