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Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in). Conventional bifocals and trifocals have visible realisticcolorsnaturaltouchopaquessophisticatedbluecontacts lines between the areas of different correction; however, lenses where the correction gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses are sometimes called progressives or no-line bifocals.To be suitable for lenses lenses, a material must be transparent, without bubbles, and have a high index of refraction. The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact resistant than glass. The advantage of glass is that it is scratch resistant and provides the clearest possible vision. Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are also offered in a variety of plastics (including acrylic, epoxy, realisticcolorsnaturaltouchopaquessophisticatedbluecontacts cellulose acetate, cellulose propionate, realisticcolorsnaturaltouchopaquessophisticatedbluecontacts polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. realisticcolorsnaturaltouchopaquessophisticatedbluecontacts Epoxy frames are resilient and return to their original shape after being deformed, so they do not need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert realisticcolorsnaturaltouchopaquessophisticatedbluecontacts to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.Fit The patient should have the distance between the eyes (PD) measured, so realisticcolorsnaturaltouchopaquessophisticatedbluecontacts that the optical centers of the lenses will be in front of the patient''s pupils. Bifocal heights also have to be measured with the chosen frame in place and adjusted on the patient. Again, realisticcolorsnaturaltouchopaquessophisticatedbluecontacts this is so the lenses will be positioned correctly. If not positioned correctly, the patient may experience eyestrain or other problems. This can occur with over-the-counter reading glasses.
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