SINGLE VISION LENSES
Single vision glasses are designed to help people who require correction of farsightedness, nearsightedness, or astigmatism. These eyeglasses have just a single optical prescription correction and they distribute focus evenly over the entire surface area of the lens. Most people that wear glasses usually get this type. If you have multiple prescriptions you would require bifocal or multifocal eyeglasses.
MULTIFOCAL EYEGLASS LENSES
Multifocal lenses technically refer to any lens that provides more than one zone of corrective power and would therefore include bifocal, trifocal and progressive lenses. If you’re in your mid 40s or older, you probably have glasses with multifocal lenses, like bifocals or trifocals. These have two or more prescriptions to correct your vision. It is designed for the many individuals within this age who struggle with presbyopia – the age-related near vision loss that requires us to use eyeglasses for reading and focusing on objects in our near vision. The multiple lens powers enable you to correct for near and distance vision with one pair of glasses.
Bifocal lenses are divided into two powers, one for distance vision and the second for near vision. Bifocals are created in a variety of designs with different sized and shaped viewing segments for near and far vision. While bifocals provide good distance and near vision, they are lacking in corrective power for intermediate areas, which is what has led to the development of trifocal and progressive lenses.
Bifocals. There have been many changes to bifocal eyeglass lenses since Franklin’s original design, making these two-power lenses thinner, lighter and more attractive. Today, the most popular bifocal 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 (or “seg”) 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 (sometimes also called a D-seg bifocals), are available in different-sized near segments and is therefore called the ST-28 (or FT-28 or D-28) bifocal. This design offers a generous field of view for reading, yet keeps the near seg small enough to be cosmetically pleasing. All bifocals, however, have a limitation.
The primary purpose of glasses bifocals is to provide the optimal balance between distance vision and near vision focusing needs. Eyeglasses bifocal lenses are available with the reading segments in a variety of shapes and widths including sunglasses bifocal options.
Though they provide good vision for distance and near, they can leave the wearer’s intermediate vision (for distances at arm’s length) blurry.