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This specifies the part of the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment. NV is an abbreviation for near vision. This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens.
Bifocals are eyeglasses with two distinct optical powers correcting vision at both long and short distances. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia , hyperopia , and/or astigmatism .
The add segment of the lens for near vision is the D-shaped area. A bifocal is a lens with two sections, separated by a line (see image to the right). Generally, the upper part of the lens is used for distance vision, while the lower segment is used for near vision. The area of the lens that caters to near vision is called the add segment.
Returning to an older prescription or different type of lens design (bifocal, trifocal) only serves to increase the adaptation period to the progressive lenses. Some wearers find the visual discomfort caused by these distortions outweigh the benefits of wearing PALs; this is known as progressive non-tolerance.
Single vision prescription lenses They can correct only one form of defective vision, either far-sightedness or near-sightedness. Multifocal lenses The multifocal lenses can correct defective vision in multiple focus, for example: near-vision as well as far-vision. This are particularly beneficial for presbyobia. [37]
Lenticular lenses are sometimes used as corrective lenses for improving vision. A bifocal lens could be considered a simple example.. Lenticular eyeglass lenses have been employed to correct extreme hyperopia (farsightedness), a condition often created by cataract surgery when lens implants are not possible.
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