(ah-met-rc -pe-ah) exists when an imperfect image is formed upon the retina, due to defective refractive power of the media, or to abnormalities of form of the eye. In myopia the anteroposterior diameter is too great, or the power of the refractive media is too great; hypermetropia (or hyperopia) is the exact reverse of the last; astigmatism is due to imperfect curvature of the cornea, or of the retina, or to inequality of refracting power in different parts of the lens; presbyopia is due to inelasticity of the lens, producing insufficient accommodation; aphakia, or absence of the lens, produces both insufficient refracting power and loss of accommodation.

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Definition of  AMETROPIA