Bifocal and Trifocal Lens Options

Just as eyeglass frames have continually changed to reflect the latest fashions, eyeglass lenses also have evolved. This is particularly true for multifocal lenses, which help those of us over age 40 deal with the normal age-related loss of near vision called presbyopia.

Benjamin Franklin, the early American statesman and inventor, is credited with creating the first multifocal lens for eyeglasses. Prior to Franklin's invention, anyone with presbyopia had to carry two pairs of eyeglasses — one for seeing distant objects and one for seeing up close.

Sometime around 1780, Franklin cut two lenses in half (one with a distance correction and one with a correction for near) and glued them together. This lens, with a line extending across the entire width of it, separated the corrections for distance and near. First called the Franklin bifocal, the lens design later became known as the Executive bifocal.

The Franklin design was further refined with a smaller, half-moon bifocal segment for seeing up close. But for many years, those who needed a correction in the intermediate zone (about arm's length away) had very few choices.

Today, however, people with presbyopia can select from many multifocal lens designs, which allow you to focus through different prescriptions for different distances. As their name implies, multifocal lenses allow you to see through more than one point of focus.

Presbyopia Computer glasses Progressive lenses

Multifocal lens advancements also have kept pace with other technologies through special prescriptions such as those that provide glasses for computer work and other tasks that take place at the intermediate range.

As their name suggests, bifocal eyeglass lenses have two lens powers—one for distance and one for near. A small portion of the bifocal eyeglass lens is reserved for the near-vision correction. The rest of the lens is usually a distance correction, but sometimes has no correction at all in it, if you have good distance vision.

Executive bifocal. The section for close vision is shown in blue. Flat-top bifocal. Round bifocal. The Executive style is one of the most common types of trifocals. The intermediate zone is shown in green. Graphics courtesy of Joe Bruneni, Vision Consultants, Inc.

The Franklin bifocal design remained in style for more than a century. Then in the early 1900s, the invention of fused bifocals offered thinner and more attractive lenses.

In a fused bifocal, a small lens segment for near vision is attached (fused) to a full distance lens. The most popular fused bifocal today has a D-shaped near segment (or "seg") rotated 90 degrees so that the flat part of the "D" is facing up. For this reason, D-seg bifocals also are called "flat-top" (FT) or "straight-top" (ST) bifocals.

Flat-top bifocals are further described by the width of the near segment, measured in millimeters (mm). The most popular FT bifocal is the FT-28, a flat-top bifocal with a near seg that is 28 mm wide.

Fused bifocals are also available in different near segment shapes for special purposes. Examples include a round seg and a narrow horizontal rectangle called a ribbon segment.

While bifocals work great for tasks like driving and reading, they are limited in their ability to provide clear vision at points in between, such as the distance to a computer monitor.

Trifocal eyeglass lenses have an additional ribbon-shaped lens segment immediately above the bifocal's near seg for seeing objects in the intermediate zone of vision — approximately 18 to 24 inches away.

This intermediate segment provides 50 percent of the magnification of the near seg, making it perfect for computer use and for seeing your speedometer and other dashboard gauges when driving.

Trifocals are especially helpful for older presbyopes — those over age 50 — who have less depth of focus than younger presbyopes, who may still be able to see objects at arm's length reasonably well through the top part of their bifocals.

As with bifocals, the most popular trifocals have a flat-top (FT) design (the flat part of a "D" that faces upward), with the near and intermediate segments being 28 or 35 mm wide.

Bifocals are typically fitted so the top line of the near segment rests at the same height as the wearer's lower eyelid. Trifocals are fitted a bit higher, with the top line of the intermediate seg placed even with the lower margin of the wearer's pupil. As bifocal wearers drop their eyes downward in order to read, the eyes naturally seek out the near-vision portion of the lens. Likewise, a trifocal wearer will focus through the intermediate segment of the lens when looking at something between 18 and 24 inches away. The eyes gravitate straight ahead, or up and over the multifocal segments, when gazing at something in the distance.

Although most bifocals and trifocals have visible lines (called "seg" lines) at the border of lens segments, there is a round-seg bifocal segment called the E-Z-2-Vue that has a less obtrusive near segment than its regular round-seg cousin. The near seg is blended into the distance portion of the lens so that it is not readily visible to the casual viewer.

Progressive lenses are special no-line multifocals that incorporate all corrections, from distance to close-up, into one lens without any separation of the various visual zones by lines. Today, more people prefer progressive lenses over bifocals or trifocals, partly because they provide a younger appearance with no age-revealing seg lines.