Lenses | Materials
There are two main types of lens designs; Single vision and multifocals.
Single vision lenses have one power across the entire surface of the lens. These lenses correct myopia (nearsightedness), hyperopia (farsightedness) and astigmatism. Single vision lenses are available in all lens materials.
Multifocals offer distance as well as near correction. The top part of the lens provides the distance refractive correction and the bottom segment provides the near refractive correction. These lenses correct presbyopia. Multifocals are broken down into 3 major groups:
Line bifocals are the most common type and least expensive and come in a variety of sizes and shapes. The most familiar type of bifocal is when the bifocal is in the shape of a sideways D. These are called flat-tops. You have a distance prescription in the top portion of the lens for far off vision and your near prescription in the bottom portion for seeing close up objects. Your Optometrist or Optician can instruct you on the benefits of each.
Line trifocals are similar to line bifocals except a portion is added to the bifocal for intermediate vision. The majority of the lens is used for distance, the intermediate portion is used for objects from about 18-26 inches (arms length) and the bifocals are used for reading and seeing objects that are within 6-18 inches. (close up)
Progressive lenses provide correction at all distances and is the closest thing to your natural eye. They offer a continuous, gradual change in prescription strength from the lower (reading correction) to the upper (distance correction) portion of the lens. Because there is no visible line, progressives have the appearance of a single vision lens and therefore more cosmetically desirable.