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 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.

lenses wearers should seek immediate attention if they experience eye pain, a burning sensation, red eyes, intolerable sensitivity to light, cloudy vision, or an inability to keep the eyes open.avoid infection, it is important for lenses wearers to exactly follow their instructions for lens insertion and removal, as well as cleaning. Soft lenses wearers should never use tap water to rinse their lenses or to make-up solutions. All lenses wearers should also always have a pair of glasses and a carrying case for their contacts with them, in case the contacts have to be removed due to eye irritation.contact can be a lot cheaper when you buy them through the mail than when you buy them through your eye doctor. Trouble is, some doctors are keeping a tight grip on lens prescriptions because they''d rather keep the profits in their practice.Rayna Brachmann, a 26-year-old legal researcher from recently switched to Biocompatible''s Proclear Compatibles lenses, which have a high water content that makes them well-suited to desert climate. But her optometrist balked when she asked if the lenses were available from another source.