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 sfx 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 theatrical it is lightweight and more impact resistant than glass. the advantage of glass grey is that sparkle 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 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 sfx 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 theatrical it is lightweight and more impact resistant than glass. the advantage of glass grey is that sparkle it is scratch resistant and provides the clearest possible vision.

preschool and school-age childrencontact are a significant improvement over lenseses for children and adolescents sfx who participate in sports or outdoor activities.contact are recommended for most athletes who need corrective eyewear. often, a single pair of lenses is less expensive than the combined cost of glasses and prescription goggles. swimmers especially can benefit from being able to see underwater while wearing regular goggles over their contact. because contact tend to dry the eye, swimmers and all athletes should take care to blink several times before and after an activity session theatrical in order to assure natural cleansing. if any pool water enters a swimmer''s eyes, the lenses should be thoroughly disinfected as soon as possible to remove pool contaminants. children who like spending time looking grey into a telescope, microscope, or binoculars may also prefer contacts over lenseses.