Just as eyeglass frames have continually changed to reflect the latest fashions, eyeglass lens technology has also evolved. This is particularly true for multifocal lenses. Multifocal lenses are designed with more than one power in the lens to help those who are over age 40 and noticing the effects of presbyopia, an age related loss of near vision.
History of Multifocal Eyeglass Lenses
Benjamin Franklin, the early American statesman and inventor, is credited with creating the first multifocal eyeglass lenses. Prior to Franklin's invention, anyone with presbyopia had to carry two pairs of eyeglasses and use one for seeing distant objects and the other one for seeing up close.
Sometime around 1780, Franklin took one lens with a distance correction and another lens with a near correction, cut the two lenses in half, and glued them together so the top half of the new lens allowed the wearer to see things far away and the bottom half helped them see up close.
This lens contained a line extending across the entire width of it, was first called the Franklin bifocal, and then later became known as the Executive bifocal.
Modern Multifocal Lenses
Bifocals. There have been many changes to bifocal eyeglass lenses since Franklin's original design. Today, the most popular bifocal design for eyeglasses is called a flat-top (FT) or straight-top (ST) design. The part that contains the power for near vision is a D-shaped segment in the lower half of the lens that is rotated 90 degrees so the flat part of the "D" faces upward.
FT or ST bifocals are available in different sized near segments. The most popular version sold in the United States has a near segment that is 28 millimeters wide, and is therefore called the ST-28 or FT-28 bifocal. This design offers a generous field of view for reading, yet keeps the near seg small enough to be cosmetically pleasing.
Other available bifocal designs include lenses with round near segments, and bifocals where the near seg extends across the entire width of the lens called an executive bifocal.
All bifocals do have their limitations. Even though bifocals provide good vision for distance and near, they can leave the wearer's intermediate vision, or distances at arm's length away, blurry.
Trifocals. Trifocal eyeglass lenses have an additional ribbon-shaped lens segment immediately above the near segment for seeing objects in the intermediate zone of vision which is approximately 18 to 24 inches away.
This intermediate segment provides good vision for computer use, seeing your speedometer and other dashboard gauges when driving.
Trifocals are especially helpful for older presbyopes who are over the age of 50 and have less depth of focus than younger presbyopes. Younger presbyopes may still be able to see objects at arm's length reasonably well through the top part of their bifocal.
As with bifocals, the most popular trifocals have a flat-top (FT) design, with the near and intermediate segments being 28 mm wide. Trifocals with 35 mm wide segments are also popular.
Limitations of Bifocals and Trifocals
Although bifocals and trifocals are functional, they do have their disadvantages. The lines in the lenses are visible. Most people prefer not to advertise their age by wearing multifocal eyeglass lenses where everyone can see the lines in them.
The lines in bifocals and trifocals cause a vision problem as well. The lines designate well defined changes in power within the lenses. As the wearer's eyes move past the lines, there is an abrupt change in how objects appear. This "image jump" can be extremely difficult for some wearers to adapt to.
Many years ago, these limitations of conventional bifocals and trifocals led to a major breakthrough in multifocal eyeglass lens design, thus creating the progressive multifocal lens.
Progressive Multifocal Lenses
Progressive multifocal lenses, also called progressive addition lenses (PALs), are true "multi-focal" lenses. Instead of having just two or three powers, progressive lenses gradually change in power from the top to the bottom of the lens. Therefore, offering a large number of powers for clear vision at all distances such as far away, intermediate, near and everywhere in between.
Another reason progressive lenses are a better choice over traditional bifocals or trifocals, is because there are no visible lines or abrupt changes of lens power in progressive lenses. Therefore, there is no "image jump". This will allow the wearer to experience more comfortable, natural vision.
With these advantages, progressive lenses have become the most popular multifocal lens sold in the United States.
The Correct Multifocal Lenses for You
With the vast number of multifocal lens options on the market today, we take time to listen, pay attention to details, and thoroughly explain your options to find a customized solution for your vision and eyewear needs. Stop by to explore your options with our highly skilled opticians, or call us at 512-255-7070 to schedule an appointment with our eye doctor to see if you are a good candidate for multifocal lenses.