Monday, May 22, 2006

23

Knapp’s Law


One problem in treating refractive errors is that the corrective lens usually changes the size of the retinal image. Many individuals can tolerate this change in image size. Problems can arise with differences in image size between the two eyes, because of asymmetric refractive errors. According to Knapp’s Law, the retinal image size will not be different between the two eyes, no matter what amount of axial ametropia exist, when the spectacle lens is placed at the eye’s anterior focal point. The front focal point of the eye is about 17 mm in front of the cornea (see schematic eye information). Preventing this from being strictly applied in clinical practice is the fact that ametropia is almost never purely axial, and a vertex distance of 16-17 mm for a spectacle correction is impractical. Most people prefer to wear their spectacles 10-14 mm in front of the cornea. Additionally, the retina in the myopic eye of a unilaterally high myope is stretched, which increases the separation of photoreceptors. This results in the effective magnification not being exactly what would be expected.



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