Bifocals, Trifocals, and Progressive Lenses

Multi-focal lenses for eyeglasses are lenses designed to allow to different fields of focus. Multi-focal lenses nowadays effectively combine exceptional functionality with stylish looks.

Types of Specialty Lenses

Specialty lenses come in three types:

1. Bifocals
2. Trifocals
3. Progressive lenses

Each of these specialty lenses has distinct features and properties that define the way the eyeglasses perform. The explanation, properties, and functioning of each type of specialty lens is described below.

Bifocal Lenses

Bifocals can be defined as eyeglasses that house two viewing zones, one for viewing objects that are far and the other for seeing objects that are near (i.e., they have two different prescriptions in the same lens). Bifocals come in different types depending on how they are used. The main motive of bifocal lenses is to achieve the most favorable balance between near vision and distant vision all in one convenient pair of glasses.

Regardless of the type of bifocal glasses preferred, all bifocals work based on the same principle. The bottom convex part of the bifocal lens is for near correction (in other words, farsighted vision) and comes in four types: round, half-moon, rectangular or half cut. The most common is the half moon, or "D" shape. The upper, least convex portion of the lens is for distant focus (nearsightedness).

Bifocals are prescribed for those who need both near and far sighted vision correction or for those who have previously had only nearsighted vision, who, usually above the age of 40, need reading glasses (farsighted help).

Trifocals

Trifocals are similar to bifocals but have three levels of prescription. They have an additional segment for intermediate vision just above the reading segment, thus allowing three ranges of magnification. This intermediate section is for focusing in the transitional position, roughly an arm's length away.

The area that resembles a semicircle in the bottom part of the lens accommodates both near and intermediate prescriptions. Then prescriptions are visually separated by two horizontal lines that run through the semicircle-like region. The near range of correction is the bottom-most part of the lens, ideal for reading and focusing on anything that is near. The mid-range of correction is approximately in the center of the lens and is most suitable for computer users. Anything beyond the mid-range focus should be focused on using the uppermost part.

Trifocals are prescribed for individuals with advanced presbyopia. This condition refers to the common loss of visual ability with age that usually sets in around the age of 40.

Progressive Lenses

Unlike bifocals and trifocals that allow only two or three ranges of magnification, progressive lenses, also commonly called no-line bifocals, incorporate multiple fields of vision into a single lens. The focus ranges shift gradually like a gradient from distance vision in the top to near correction range at the bottom, smoothly including all the in-between ranges. The lens also does not have any visible lines that separate different fields of vision. The result of vision in a progressive lens is closest to natural vision.

It takes a while for one to get used to progressive lenses. The changes that occur at the edge of the lenses slightly alter the peripheral vision. Some find it difficult to get used to the gradual shifting ranges of vision of the progressive lenses. However, these discomforts with progressive lenses typically diminish with regular wear.

Progressive lenses, like trifocals, are usually prescribed for those with advanced presbyopia.

Comparison of Bifocals, Trifocals, and Progressive Lenses

Even though bifocals serve the purpose of two glasses effectively in one, they have major disadvantages, like the abrupt jump of images when the visible line that separates the two layers of lenses is passed and no intermediate vision is available.

Trifocals overcome the drawback of the absence of a third region of vision by providing an intermediate layer. However, this makes it more difficult to adjust to, due to the distinct lines that separate each field.

Progressive lenses thus have various benefits over bifocals and trifocals because they are designed to try to imitate the action of human eyes. Unlike bifocals and trifocals, progressive lenses are multi-focal lenses that enable gradual focus points from far to near. With progressive lenses, it is possible to look at any field of vision—distant, intermediate, and near—without any struggle to adjust your vision or an abrupt jumping of images.

However, even progressive lenses have their disadvantages, including skewed peripheral vision and dizziness or nausea for those not yet used to the gradient lens. Also, if the frame’s length is short, the reading portion of the progressive lens will have to be cut short, limiting some frame selections.

Conclusion

Multi-focal lenses, though having all both advantages and disadvantages, overcome all drawbacks by efficiently aiding those with multiple vision difficulties. Multi-focal lenses do a lot to treat vision-based disorders and are extensively prescribed for use worldwide.

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