Refraction error
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Refraction error is an error in the focusing of light by the eye. There are two main types: spherical errors and cylindrical errors. These can be corrected by glasses,contact lenses, or refractive surgery.
Spherical errors occur when the optics of the eye (the dioptrics) are either too powerful or too weak to focus light on the retina. It is as if the overall lens is more or less spherical than it needs to be. People with this refraction error see all contours as blurred. When the optics are too powerful for the length of the eyeball (this can arise from a cornea with too much curvature or an eyeball that is too long), one has myopia. When the optics are too weak for the length of the eyeball (this can arise from a cornea with not enough curvature or an eyeball that is too short), one has hyperopia.
Cylindrical errors occur when the optics of the eye are too powerful or too weak across one meridian of the optics. It is as if the overall lens tends towards a cylindrical shape along that meridian. People with this refraction error see contours of a particular orientation as blurred, but see contours with orientations at right angles as clear. When one has a cylindrical error, one has astigmatism.
Other refraction errors include keratoconus.