discomfort. Another criticism of over-the-counter glasses is that they may alleviate symptoms, causing a person to forgo the other benefits of routine vision exams, such as early diagnosis of chronic disease. Self-selected corrective lenses Although lenses are normally prescribed by optometrists or ophthalmologists, there is evidence from developing countries that allowing people to select lenses for themselves produces good results in the majority of cases and is less than a tenth of the cost of prescription lenses. Lens types Single vision Single vision lenses correct for only one distance. If they correct for far distance, the person must accommodate to see up close. If the person cannot accommodate, they may need a separate correction for near distances, or else use a multifocal lens (see below). Reading glasses are single vision lenses designed for near work and include over the counter glasses. They come in two main styles: full frames, in which the entire lens is made in the reading prescription, and half-eyes, style glasses that sit lower down on the nose. Full frame readers must be removed to see distance clearly, while the distance can be clearly viewed over the top of half-eye readers. Bifocal A bifocal is a lens with two sections, separated by a line (see image to the right). Generally, the upper part of the lens is used for distance vision, while the lower segment is used for near vision. The area of the lens that caters to near vision is called the add segment. There are many different shapes, sizes, and positions for the add segment that are selected for functional differences as well as the visual demands of the patient. Bifocals allow people with presbyopia to see clearly at distance and near without having to remove the glasses, which would be required with single vision correction. Trifocal Trifocal lenses are similar to bifocals, except that the two focal areas are separated by a third area (with intermediate focus correction) in the middle. This segment corrects the wearer's vision for intermediate distances roughly at arms' length, e.g. computer distance. This lens type has two segment lines, dividing the three different correcting segments. Progressive Progressive addition or varifocal lenses provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing clear vision at all distances, including intermediate (roughly arms' length). The lack of any abrupt change in power and the uniform appearance of