knapps law and unilateral axial high myopia 1970 background and purpose hermann knapps 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 lawmethods and subjects 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 ascan ultrasonic echography axial biometryresults 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 12 mm with reasonable correlations between axial length differences and both clinical ametropia anisomyopia and the amount of aniseikonia foundconclusions knapps 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