Corrective lens
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A corrective lens is a prosthetic lens worn on or before the eye, used to treat myopia, hypermetropia, presbyopia and astigmatism. The most common types of corrective lens are spectacle lenses and contact lenses. Myopia (short sightedness) requires biconcave or diverging lenses, whereas hypermetropia (long sightedness) requires biconvex or converging lenses. Correction of presbyopia needs converging lenses in the case of a person with or without long sightedness.
In some cases, mild farsightedness can be treated with simple magnifying lenses or "reading glasses". However, an optometrist may need to prescribe a dioptre for both eyes or each eye, allowing lenses to be custom ground to the patient's specific needs. Usually, the dioptre (or "prescription") is equal for both eyes; in some cases, however, the two eyes may need different prescriptions.
Some patients benefit from bifocals, or lenses with separate sections ground to different prescriptions for different circumstances. Typically a person with myopia would have one section of a prescription lens that has a certain diverging power while another section of the lens would have a lower diverging power for close-up work. Similarly a person with hypermetropia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.
It is essential for those who wear eyeglasses or contact lenses to consult their optometrists regularly to ensure that their prescription remains constant, especially in the case of children.