another type of multifocal, theatrical a trifocal, crazy has an area in-between that corrects for intermediate distances wolf (usually about 28 in). conventional black bifocals and trifocals have visible lines between the areas of different correction; however, lenses contact where the correction gradually changes from one area to the other, without visible lines, have been available since the lenses 1970s. such lenses are sometimes called progressives or no-line bifocals.to be suitable for sfx lenses lenses, a material must be transparent, without theatrical 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 crazy 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 type of multifocal, theatrical a trifocal, crazy has an area in-between that corrects for intermediate distances wolf (usually about 28 in). conventional black bifocals and trifocals have visible lines between the areas of different correction; however, lenses contact where the correction gradually changes from one area to the other, without visible lines, have been available since the lenses 1970s. such lenses are sometimes called progressives or no-line bifocals.to be suitable for sfx lenses lenses, a material must be transparent, without theatrical 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 crazy 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 wolf offered in a variety of plastics (including acrylic, epoxy, cellulose acetate, cellulose propionate, black polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. 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 to their original shape after extreme trauma and distortion; because of this property, though, they contact cannot be readjusted after they are manufactured.fit the patient should have the distance between the eyes (pd) measured, so that the optical centers of the lenses 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. sfx again, this is so the lenses will be positioned correctly. if not positioned correctly, the patient may experience theatrical eyestrain crazy or other problems. this can occur with over-the-counter reading glasses.