• Single Vision lenses are the standard eyeglass prescription. These glasses are for one field of vision across the entire lens, whether specifically for distance vision, near-vision reading glasses or somewhere in-between such as computer-distance glasses. There are no lens height requirements for single vision prescriptions.
• Bifocals are lenses with two fields of vision separated by a line. Usually the top is designated for distance-vision or computer-distance and the bottom for near-vision work such as reading. The bottom reading area is 28mm wide and 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. The lens height for a Bifocal lens must be 30mm or greater.
• Progressive lens design includes three fields of vision, without lines, and is sometimes referred to as a "no-line bifocal". In a Progressive lens, the shape of the corrected part of the lens is approximately that of a funnel or a mushroom, with the top portion for distance-vision, narrowing to the lower middle for mid-range (corridor), finally to the bottom portion for the near-vision or reading. The reading area is approximately the lower quarter, central third of the lens. The height of the progressive corridor can be short, medium or long, and is dependent on the height of the lens: the taller the lens, the longer the corridor. The corridor begins just above the centerline of the lens. The lens height for any Progressive lens must be 30mm or greater.