Bifocal

A bifocal lens. Russell Burden

 

A bifocal is a lens that has two focal points, usually one lens for viewing distant objects, and one lens for viewing close objects. A bifocal is also known as a multi-focal lens. Ben Franklin, is credited, as the inventor of the original bifocal lens.

Why do some people need a bifocal?

 

Presbyopia 

It is difficult to understand it, until you reach that time in your life, but around age 40, we develop presbyopia.

 Presbyopia causes the gradual decrease in the ability to focus on near objects and reading small print. At first, people notice difficulty reading in darker environments and reading medicine bottle size print. People with presbyopia also notice that it is more difficult to focus from distance to near. As more time goes by and they get closer to age 42-45, it starts to become a more dominant issue in their life and everything they try to read or look at in a close range becomes extremely difficult. Doctors don't understand the process exactly but they believe presbyopia develops because of a combination a muscle (ciliary body) inside eye not working as well and the lens located inside our eyes, becoming more and more inflexible. The sum effect is the lack of ability to focus on near objects. The answer to this problem is for your optometrist to prescribe a bifocal. At this point, we need one part of the lens to focus at distance and one part to focus at near.

Accommodative Dysfunction

Some people need a bifocal is because of an accommodative dysfunction. Some children develop a condition where they are unable to focus easily from distance to near. They also experience great fatigue while trying to maintain focusing at near during reading or learning in the classroom.

 

Accommodative Esotropia

Another reason why some children must wear a bifocal is because of a condition called accommodative esotropia. Accommodative esotropia is one of the most common forms of strabismus. Usually, these children are very farsighted. These children have an esotropia or crossed eye caused by the focusing efforts of the eyes as they attempt to clearly and compensate for the farsightedness. They have an imbalance between the eye muscle converging system and the focusing system. A bifocal often alleviates the problem totally.

Are there different types of bifocal lenses?

Bifocal lenses are available in several different types.

  • Flat-top - A flat top bifocal has a small segment shaped like a half moon or the letter "D" on its side. It is positioned nasally and down toward the bottom of the lens. Depending on preference, a patient may choose between a 25 mm segment or a 28 mm segment. The top of the segment has a visible line in the lens.
  • Round seg - A round bifocal is a 22 or 24 mm round segment toward the bottom of the lens. This bifocal was developed to allow the wearer to reach the reading area of the segment a little easier. Round segments are not as commonly worn today.
  • Blended - a blended bifocals were popular before no-line bifocals were developed. They are typically made of a round segment where the edges are blended in with the rest of the lens. While a trained optician can easily see it, to the rest of the world it is much less visible than a flat top bifocal.
  • Executive - An executive bifocal is a line bifocal where the top half (distance) part of the lens is separated completely from the bottom half (near) part of the lens with a visible lens that extends all the way across the lens. These were very popular with people that did a lot of office and desk work for their occupation and was thought to give them more room in which to focus. They have become more unpopular in recent years because of the unsightly line that goes all of the way across the lens. Also, optical professionals learned that people really did not use most of the lens in the temporal area as much as one thought and a flat-top bifocal performed just as well.
  • Progressive - Progressive lenses are lens that you may hear advertised as an invisible, seamless or no-line bifocal. Progressive lenses have a gradual increase or transition in reading power as the lens extends down toward the bottom of the lens without any visible lines. Instead of having a blunt cut-off from distance to near, there is a gradual change in power. Progressive lenses because popular quickly because they give a more youthful appearance.

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