Refractive Errors or Types of Vision Problems
People who are nearsighted (myopia), farsighted (hyperopia), or have astigmatism (shape of the eye is like a football) will all develop Presbyopia. People who need no glasses to see or emmetropia will also develop Presbyopia. Many people are confused by hyperopia and emmetropia. Most people believe that someone who is farsighted does not need glasses to see at distance. The actual word for not needing glasses to see is emmetropia. People who are farsighted need glasses to see clearly at distance. Their vision is better at distance than near without glasses and that is where the confusion comes about.
What is Presbyopia?
Presbyopia occurs as we enter into our forties. The lens inside the eye located behind the pupil focuses images onto the retina in the back portion of the eye. As we focus, the muscles that are attached to lens by zonules (tiny ligaments) change the shape of the lens so that we can focus from distance to up close. As we enter our forties, everyone develops presbyopia or the inability to focus from distance to near. Through the forties and fifties the lens gradually loses its ability to change its shape as the material inside the lens ages (becomes stiff and less pliable). As we get into our sixties, we have lost nearly all our ability to focus. Even someone who is nearsighted will not be able to see at near while wearing their distance glasses. They can remove their glasses and see up close.
Glasses for Reading
Someone who is emmetropic may choose not to wear bifocal but instead wear reading glasses or cheaters that you can buy without a prescription. People who have a prescription for distance will need to decide on what type of glasses to wear either bifocals or trifocals.
There are two types of bifocals. Bifocals glasses can either have a line that can be seen or a progressive bifocal which does not have a line.
Bifocals with a Line
People who wear this type of bifocal have two areas of clear vision distance and at one area at near. The area of clear vision at near depends on the strength and the individual’s age. As we age, the lens gradually looses its ability to focus. Therefore, the bifocal prescription has to be stronger as we age. As the bifocal becomes stronger, the depth or area of clear vision becomes smaller. This progresses to the point that there is no intermediate vision with a lined bifocal.
As people lose their intermediate vision, some people may choose to wear a trifocal in order to see clearly at an intermediate distance. Trifocals have two visible lines in the lenses. The top portion of the trifocal glasses is for distance. The middle portion of the glasses is for intermediate vision. The bottom portion of the glasses is for up close vision. Many people have trouble adjusting to trifocals and using the different portions of the lens. For people who do adjust to the trifocal lens, it provides them a greater range of near vision. Fewer people are wearing trifocals today because of the increased popularity of progressive or no-line bifocals.
Progressive or No-Line Bifocals
A progressive bifocal does not have a visible line and gradually strengthens as a person looks down through the lens. This gives the person the ability to seamlessly see from distance, to intermediate, and near. In the past, this blending of distance to near vision had problems with distortion and narrow area of clear vision for near. New technology has vastly improved the quality of the blending and nearly 95 % of people adjust to the lens.
Quality of Progressive Lenses
There are more than 100 companies that make progressive bifocals. There are significant differences between the brands of progressive bifocals. The top of the line companies provide a much wider area, have clearer vision, and have less distortion than the lower level of lenses. Many of the lesser quality lenses are difficult to adjust to, the progression is not good, the peripheral vision seems distorted, and people have the feeling or sensation of not seeing well. It is very important that you buy a top quality lens in order for you to have the best possible vision.