Knapp’s law

Knapp’s law, also called Knapp’s rule, states that in axial ametropia a correcting lens placed at the anterior focal plane of the eye forms a retinal image the same size as in an emmetropic eye.^1,2 The anterior focal point lies roughly 14 mm from the eye, close to where a spectacle lens sits, so spectacle correction of axial ametropia gives a retinal image near emmetropic size.^1,3

In anisometropia, two eyes with similar optical components that differ only in axial length get nearly equal retinal images from spectacles, and unequal images from contact lenses or corneal refractive surgery.¹ The reverse holds for two eyes that differ only in corneal power, where contact lenses or refractive surgery tend to equalize image size and spectacles make the images differ.¹

Knapp’s law is geometrically correct, yet it does not hold in every patient.² Aniseikonia can follow spectacle correction of axial anisomyopia, because stretching of the posterior pole in high myopia lowers retinal receptor density and causes a perceptual micropsia despite equal retinal images (basic aniseikonia).²