Knapp's law and unilateral axial high myopia. 1970

(Hermann) Knapp's Law states, in essence, that in axial ametropia, if the correcting lens is placed so that its second equivalent point coincides with the anterior focus of the eye (ca 15 mm in front of the eye), the size of the retinal image is not different from that in the normal eye. The advent of ocular ultrasonic echography and axial biometry permits us to now test this Law.

A "Direct Comparison" Eikonometer was devised using the Aulhorn Phase- Difference Haploscope, projectors, and half circle targets with equalized luminance. Five patients with anisomyopia were tested for aniseikonia after complete ophthalmologic examination including also A-Scan ultrasonic echography axial biometry.

Significant aniseikonia was found in all five subjects ranging from 10% to 60% angular minification in the myopic eye. Ultrasonic axial biometry showed axial length differences from 2 to 5 1/2 mm with reasonable correlations between axial length differences and both clinical ametropia (anisomyopia) and the amount of aniseikonia found.

Knapp's Law may be optically right but clinically wrong. That is, its clinical application, which predicts the absence of aniseikonia in axial aniosometropias is not fulfilled. Retinal stretching due to myopia and resultant retinal micropsia is hypothesized to be the cause of this optically unexpected aniseikonia. The retinal images may be isometric but the patient will still have significant cortically perceived or perceptual aniseikonia due to retinal microspia. Such patients may benefit from contact lenses or other aniseikonic corrections.