Fig. 5. The principle of effectivity. The lens power needed to correct a refractive error depends on the vertex distance of the lens. A -10.00-D lens at a vertex distance of 10 mm corrects the refractive error because the back focal point of the lens falls at the far point of the eye (top). A lens positioned at a vertex distance of 15 mm must have a power of -10.50 D to correct the same refractive error (bottom).