Bifocals are lenses with two distinct corrective powers. Bifocals are commonly prescribed to presbyopes who require a correction for myopia (nearsightedness) or hyperopia (farsightedness) with or without correction for astigmatism (distorted vision as a result of irregularly shaped lens or cornea. The primary purpose of a bifocal lens is to provide the optimal focus balance between distance and near vision.
Generally, you look up and through the distance portion of the lens when focusing on points farther away, and you look down and through the bifocal segment of the lens when focusing on reading material or objects within 18 inches of your eyes
It is generally accepted that Benjamin Franklin invented the bifocal. The most common bifocal today is the Straight Top 28 Bifocal which has a straight line across the top with a 28mm radius. There are several varieties of straight top bifocals available today including: Straight Top 25, Straight Top 35, Straight Top 45 and the Executive (The original Franklin Seg) which runs the complete width of the lens.
In addition to straight top bifocals there are completely round bifocals including Round 22, Round 24, Round 25 and Blended Round 28 (no definitive segment). The advantage to the round segment is that there is less image jump as one transitions from the distance to the near portion of the lens.
Occupational bifocal lenses are designed for performing a particular job or hobby and are not meant for general-purpose wear.
One popular occupational bifocal lens is the “Double-D.” This lens has an upside-down flat-top segment for near or intermediate vision in the top third of the lens and a second flat-top segment for near vision in the bottom third. The segments are commonly offered in 28mm and 35mm varieties.
The Double-D occupational lens is great for car mechanics, who need to see their tools and other near objects, but also need to work on vehicles on a lift above them. The upper segment helps them see the undercarriage of the vehicle without having to tilt their head far backward. Lens Pricing