shape magnification

The part of spectacle magnification that comes from the form of the lens rather than its power. A lens enlarges or shrinks the retinal image through its front-surface curvature, its center thickness and its refractive index, and this contribution is called the shape factor¹.

The shape factor is $M_s = \dfrac{1}{1 - \dfrac{t}{n}F_1}$, where $t$ is the center thickness in meters, $F_1$ the front-surface power in diopters and $n$ the refractive index¹. Of two lenses of the same power, the one with the more curved front surface or thicker center magnifies more¹. A lens with a plus front surface and a minus back surface that add to zero power still magnifies, because it behaves like a tiny telescope^1,2.