Corrective lens




A corrective lens is a lens typically worn in front of the eye to improve vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal, but can be used for purely refractive purposes.

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Vertex distance

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Vertex distance is the space between the front of the eye and the back surface of the lens. In glasses with powers beyond ±4.00D, the vertex distance can affect the effective power of the glasses. A shorter vertex distance can expand the field of view, but if the vertex distance is too small, the eyelashes will come into contact with the back of the lens, smudging the lens and causing annoyance for the wearer. A skilled frame stylist will help the wearer select a good balance of fashionable frame size with good vertex distance in order to achieve ideal aesthetics and field of view. The average vertex distance in a pair of glasses is 12-14mm. A contact lens is placed directly on the eye and thus has a vertex distance of zero.
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Lens materials

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This section needs additional citations for verification . Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( January 2015 ) (Learn how and when to remove this template message) Optical crown glass (B270 - Soda/lime glass) edit Refractive index (n d ): 1.52288 Abbe value (V d ): 58.5 Density: 2.56 g/cm3 (the heaviest corrective lens material in common use, today) UV cutoff: 320 nm Glass lenses have become less common owing to the danger of shattering and their relatively high weight compared to CR-39 plastic lenses. They still remain in use for specialised circumstances, for example in extremely high prescriptions (currently, glass lenses can be manufactured up to a refractive index of 1.9) and in certain occupations where the hard surface of glass offers more protection from sparks or shards of material. If the highest Abbe value is desired, the only choices for common lens optical material are optical cr...
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Prescription of corrective lenses

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Corrective lenses are typically prescribed by an ophthalmologist or an optometrist. The prescription consists of all the specifications necessary to make the lens. Prescriptions typically include the power specifications of each lens (for each eye). Strengths are generally prescribed in quarter-diopter steps (0.25 D) because most people cannot generally distinguish between smaller increments (e.g., eighth-diopter steps / 0.125 D). The use of improper corrective lenses may not be helpful and can even exacerbate binocular vision disorders. Eyecare professionals (optometrists and ophthalmologists) are trained to determine the specific corrective lenses that will provide the clearest, most comfortable and most efficient vision, avoiding double vision and maximizing binocularity. Over-the-counter correction edit Ready-made single-vision reading glasses go by many names, including over-the-counter glasses, ready readers, cheaters, magnifiers, non-prescription readers, or generic readers. T...
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