Presbyopia
Bifocal lenses and glasses are prescribed to correct a condition called presbyopia. People with presbyopia have problems seeing up close and at a distance. Rather than having two pairs of prescription glasses, one for near viewing and one for distance viewing, bifocal glasses and lenses can address both problems at once. In both conditions, the problem is that rays of light entering the eye do not focus properly on the retina. The goal of corrective lenses is to compensate for this, properly focusing incoming light rays so that the vision correction allows you to see as close to perfect as possible.
Bifocals
A bifocal lens is a single lens with two separate sections each having a different prescription. The upper part of the lens is designed for distance vision, while an inset portion at the bottom is designed for up close vision. Depending on your needs, the inset may be a small circle, a small half-circle, a small rectangle or the entire bottom half of the lens. As you look forward into the distance, your eye naturally looks through the upper part of the lens, and your distance vision is corrected. As you look down to read or perform other close work, your eye naturally looks through the lower part of the lens, and your near vision is corrected.
Other Bifocals
In executive bifocals, the lower near vision segment extends all the way across the bottom half of the lens. This is an asset for people who need a wider field of near vision, accountants for example, but produces a heavier pair of glasses with a noticeable demarcation between sections. Double segment bifocal lenses have near vision insets at the top and bottom of the lens. These are useful for people who need to see up close while looking up as well as down, construction and library workers, for example. For cosmetic reasons, some people choose blended segment lenses with no noticeable line of demarcation between the two prescriptions. Progressive lenses, in which the prescription changes gradually from top to bottom, also have no line; it usually take longer to become comfortable with progressive lenses than traditional bifocal lenses.
