AMETROPIA. The normal refractive condition of the eye is known as emmetropia. Any departure from this condition is called ametropia. Like a camera, the eye is provided with a lens and a light-sensitive membrane, the retina. cornea and the transparent substances which fill the eyeball aid the refractive power of the lens. In an ideal eye at rest, parallel rays come to a clear focus on the retina. Although only about 2% of properly examined eyes exactly meet this standard, the majority of normal eyes closely approach it. When, however, due to defects in the refractive mechanism, parallel rays do not come to a clear focus on the retina of a resting eye, the condition is called ametropia. Such defects may be abnormalities in length of the eyeball or irregularities in the curvature of the cornea or lens, and the resulting varieties of ametropia are called myopia, hyperopia and astigmatism. (See ASTIGMATISM. )