Single Vision lenses are the standard eyeglass prescription. These glasses are for one field of vision across the entire lens. Single vision glasses can be prescribed for distance vision, near-vision reading glasses, or computer-distance glasses. There are no lens height requirements for single vision prescriptions.
Progressive lenses includes three fields of vision, without lines. In a Progressive lens, the shape of the corrected part of the lens is approximately that of a funnel or mushroom. The top portion is used for distance-vision, the middle portion (corridor) for mid-range vision, and the bottom portion for the near-vision or reading.
Bifocals lenses include two fields of vision separated by a line. Usually the top is designated for distance vision or computer-distance vision, and the bottom for near-vision work such as reading. The bottom reading area is positioned just below the centerline of the lens. The position of the bi-focal area will be affected by the height of the lens chosen.