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A refractive error is a disorder which interferes with the eye's ability to focus light, causing vision problems. There are a number of different types of refractive errors, and they can be caused by genetic and environmental causes. These disorders are typically treated by a doctor who specializes in eye care, such as an ophthalmologist, and they can be corrected with the use of specialized lenses which compensate for the problems with the structure of the eye.
The eye is a delicately tuned organ which is designed to gather and focus light. In someone with no refractive errors, the focal point of the light is precisely targeted to hit the retina, generating a crisp and clear image which can be interpreted by the cells in the retina and then relayed to the brain.
In someone with a refractive error, a problem with the structure of the eyeball or the cornea causes the focal point to move, which results in vision problems. In myopia or nearsightedness, the focal point is located in front of the retina, forcing the person to move closer to objects to see them more clearly. Hyperopic or farsighted people have the opposite problem, with a focal point behind the retina which means that they need to be further away from objects to see them.
In astigmatism, the structure of the cornea is not even, resulting in a scattered focal point and blurry vision at any distance. People can also develop a type of refractive error known as presbyopia, in which the eye is less flexible over time, making it harder to read things up close.
Using a lens such as a contact lens or a set of glasses, it is possible to compensate for a refractive error within the eye which is causing vision difficulties. It is also possible to overcorrect to make someone farsighted or nearsighted, which is sometimes done deliberately and sometimes done by accident. People can see the effects of overcorrection by wearing a set of glasses with a stronger prescription than their own, or by donning glasses when they do not normally wear them.
When an ophthalmologist examines someone with a refractive error, he or she comes up with a prescription expressed in diopters which indicates the type of lens needed. People with myopia need concave lenses, which have a negative value such as -1.5, a common prescription, while hyperopic individuals need convex lenses with a positive diopter value like +3.