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Difference between Bifocals and Trifocals

Bifocals vs Trifocals

Bifocals and trifocals are both eyeglasses. They come with the added benefit that the same glasses can be used to view objects at different distances. They differ in the number of optical power regions they have. Bifocals, as the name suggests, have two regions of optical power. Trifocals, on the other hand, have three different regions of optical power.

Bifocals have a lens that is divided into two parts. The lower part is used to view things near to the person’s eyes whereas the upper part is used to view things that are distant. Trifocals contain a lens that has an additional region apart from the two that are already present in bifocals. This is an intermediate region that is used to view things that are typically an arm’s length away. For example, this region comes in handy when working on a computer.

Bifocals were invented in the 18th century by Benjamin Franklin. Trifocals on the other hand, were developed much later in the 1940s. Initially bifocals were made simply by joining two different lenses together. Because of this the early glasses developed in such a manner were quite fragile. No such thing was done for the trifocals.

Trifocals and bifocals are usually prescribed to people with presbyopia. This is the usual loss of visual accommodation that is accompanied with age. However, bifocals are also prescribed to children whereas trifocals, generally, are not.

Bifocals have a fine line where the two types of optical powers meet. On the other hand, trifocals do not have this line but an intermediate region. This also leads to distortion in trifocals which is not present when one is wearing bifocals.

Bifocals often cause dizziness which can result in headaches of the person wearing them. This is because it is often difficult to get accustomed to the way in which they are used. For this purpose, trifocals were created. Their intermediate region usually does not cause dizziness or headaches.
Although trifocals are used for intermediate distances, the area that covers the intermediate distance is very small. Sometimes it is preferable to order customized bifocals whose far vision area is adjusted to the approximate distance between the user and his working environment, say, between a user and a computer. However, such bifocals cannot be used all the time as they will lack having distant vision.

Another notable difference between trifocals and bifocals is that there is an image jump associated with bifocals as one lifts his or her eyeballs. This is because the angle under which one views an object significantly changes. Contrarily, in a typical trifocal the angle changes gradually as one lifts his or her eyes due to the presence of the intermediate area.

Both bifocals and trifocals are used in accordance with the needs of the person. They save the user the hassle to carry multiple pair of glasses all the time. However, the features of the two pairs differs.

Summary:

Bifocals have two regions of optical power, trifocals have three.
Bifocals are used for near and distant vision whereas trifocals are used for near, intermediate and distant vision.
Bifocals were invented much earlier than trifocals.
Bifocals and trifocals are usually prescribed to adults experiencing presbyopia.
Bifocals are also prescribed to children, trifocals are not.
Trifocals can cause distortion.
Bifocals can lead to headaches as compared to trifocals.
Trifocals have very small intermediate distances. Sometimes its preferable to use a customized bifocal instead.


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