another sfx type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 theatrical 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 smiley progressives or no-line bifocals.to be suitable for lenses lenses, a material must be transparent, without bubbles, and have a high index of sfx 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.

another sfx type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 theatrical 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 smiley progressives or no-line bifocals.to be suitable for lenses lenses, a material must be transparent, without bubbles, and have a high index of sfx 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.