There are two different types of eyeglass lens designs: single vision, an all-purpose lens designed to correct distance vision, and multifocal, designed to correct both distance vision and near vision (the upper portion is focused for distance vision, while the bottom portion is used for up close activities such as reading).
Multifocal lenses are eyeglasses used to correct presbyopia, the eye's natural diminished ability to focus on near objects that comes with age. Bifocals have a correction for reading on the bottom half of the lens and another for seeing at a distance on the top. Some specialized lenses may also have segments at the top for those who need to look upward at objects that are in the intermediate or near range (double-D bifocals). Trifocals are lenses with three different lens corrections — distance vision, intermediate vision, and near vision — in one set of eyeglasses. Progressive lenses function generally the same way as bifocals or trifocals; however, they have a smooth transition between distance and near focal areas instead visible dividing lines. While the invisible transition of progressive lenses may be more aesthetically pleasing, the focal areas are relatively small because more lens space is used for the transitional areas. Progressive lenses cause more distortion than other types of lenses, making them more difficult to wear for approximately 10 percent of the population.