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 theatrical lines between the areas of different manson 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 sfx advantage of glass is that it is scratch resistant and provides the clearest possible vision.

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 theatrical lines between the areas of different manson 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 sfx advantage of glass is that it is scratch resistant and provides the clearest possible vision.

simultaneous lenses bring distant and near objects into focus at the same time. the center of the lens corrects distance vision; the outer part corrects near vision. * segmented lenses correct distance vision in the top half and near vision in the bottom half.monovision an alternative to bifocal contacts, monovision lenses correct distance vision in one eye and near vision in the other. modified monovision allows wearers to put a bifocal lens in one eye and a single-vision lens in the other.implants designed for mild nearsightedness, keravision intacs are placed in the periphery of the cornea by an ophthalmic surgeon. made of polymer, a type of hard plastic, they require no maintenance and can be surgically removed. the outpatient procedure has a 97-percent success rate of 20/40 vision or better.