The aging process includes changes in vision that makes everyday reading more difficult than usual because of the loss of one’s ability to focus naturally. This condition is called presbyopia. It is often relieved by wearing special reading glasses. People who have been wearing prescription eyeglasses because of nearsightedness or farsightedness will need bifocals or eyeglasses with two different lens powers to help them see objects at different distances.
Some presbyopes opt for progressive multifocal lenses, which gradually change powers from the top portion to the bottom of the lens, without lines to separate them. However, conventional bifocals offer some advantage over progressive lenses, such as providing wider lenses for computer work and reading compared to progressive lenses. Special purpose bifocals are also available for computer work and other tasks requiring powerful near and intermediate vision.
Functions of Bifocals
Bifocals have two different lens powers that aid in seeing objects at various distances once the aging process reduces your ability to do so. These are usually prescribed to adults older than 40 years, although some children and younger adults may also need them for focusing problems, which strain the eyes while reading.
The bottom half of bifocal lenses reduce the effort needed to focus to be able to see nearby objects more clearly. Research also suggests that wearing bifocals as well as other multifocals may help reduce focusing demands in children with myopia (nearsightedness) while they read or do near work.
Whatever the reason one might need to obtain prescription for correction of near-vision, bifocals work similarly. That is, the small lower lens segment (also called "seg") has the refractive power you require to correct near vision while the other part of the lens works for distance vision.
The "seg" is available in various shapes:
- Half-moon or flat-top, D segment, or straight-top
- Round segment
- Ribbon segment, which is a thin rectangular area
- Franklin/Executive/E style, which is the bottom portion of the bifocal lens
In using bifocals, you would look through the upper part of the bifocals to focus on farther objects and look down through the lower segment to read or focus on objects at a distance of 18 inches or less.
How Are Bifocals Fitted?
Your bifocals are typically fitted so that the line between the upper and lower portions rests at the same level as your lower eyelid. As you drop your eyes to read downward, your eyes will naturally use the segment for near-vision.
Bifocals have noticeable lines between the upper and lower portions but those in round-seg bifocals tend to be less conspicuous than those of Executive and flat-top styles. Some lenses have "invisible bifocals," which are round-seg bifocals with the visible lines buffed out. However, this may cause some optical distortion. If you prefer to wear bifocal lens without the visible lines, your best option is to use progressive lenses.
Occupational bifocal lenses have been designed for use in particular jobs or hobbies rather than for general purposes. These specialty lenses are intended to solve specific vision problems so you must tell your eye doctor about your special needs for work or hobby.
1. Double-D multifocal lens
This is a popular type of occupational multifocals that has three segments. The upper third is a flat-top segment that is upside-down and used for near vision (for bifocals) or intermediate vision (for trifocals). There is a second flat-top seg in the bottom portion for near vision. The center is for your distance vision.
This type of lenses is useful for many car mechanics or workers who need their vision to see tools and near objects, and also need to see the underside of vehicles overhead on lifts, that is, without tilting their heads far backward nor using their bifocals upside down.
2. Golfer's bifocals
Older golfers have a special type of bifocals where the small round segment for near vision is placed in the outer lower corner of just one of the lenses. If the golfer is right-handed the seg is on the right lens and on the left if he is left-handed.
3. Other types
Other types of multifocal lenses feature trifocals such as the E-D trifocals with an Executive intermediate segment at the bottom of the lens as well as a small flat-top segment for near vision set within an intermediate seg.
For your special needs, it is best to consult your eye care practitioner to determine which type of bifocals or multifocal lenses are suitable.