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A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity. The number of people globally with refractive errors has been estimated from 800 million to 2.3 billion.〔 ==Classification== An eye that has no refractive error when viewing distant objects is said to have ''emmetropia'' or be ''emmetropic'' meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object 8 meters or further away from the eye. This proves to be an evolutionary advantage by automatically focusing the eye on objects in the distance because it allows an individual to be alert in, say, a prey-predator situation. An eye that has refractive error when viewing distant objects is said to have ''ametropia'' or be ''ametropic''. This eye cannot focus parallel rays of light (light from distant objects) on the retina, or needs accommodation to do so. The word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors: * Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error frequently have blurry vision. * * Myopia: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can easily be corrected with a concave lens which causes the divergence of light rays before they reach the cornea. * * Hyperopia: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). This can be corrected with convex lenses which cause light rays to converge prior to hitting the cornea. * Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder. * *Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose. * *Presbyopia: When the flexibility of the lens declines typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「refractive error」の詳細全文を読む スポンサード リンク
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