Posted on September 03 2015
Besides lens material, another important factor in selecting lenses is the style of lens. While the material and size of the lenses will depend on the lifestyle and look a person wants, lens style will mostly be based upon their prescription. An eye-care professional will help a person select the right style of lens. Some of the most common lens styles are single-vision, bifocals, trifocals and progressives.
Single-vision lenses are a group of lenses that treat one distance. Both plus and minus lenses are considered single-vision. This makes sense because both of those types of lenses treat either farsightedness (plus) or nearsightedness (minus). Sphero-cylinder lenses, used to treat astigmatism, are also grouped as single-vision. Although sphero-cylinder lenses have two components – the spherical side used to treat refractive errors and the cylinder side used to treat meridian discrepancies – they are still uniform strength across the whole lens. This separates regular sphero-cylinder lenses from bifocals and progressive lenses.
Bifocals are lenses with two separate areas of differing optical strength. Generally, these types of lenses are recommended to people who have developed presbyopia. As we discussed in the article on this condition, presbyopia is an age-related condition that causes the eye to lose focusing power in close-up vision. Not all people with presbyopia need bifocals. However, they are necessary for someone who wore glasses for farsightedness, nearsightedness or astigmatism and then developed presbyopia. One segment of bifocals treats the original condition and another small segment treats the presbyopia. Another way to think of bifocals is that they are like single-vision lenses with a section for close-up reading (classified as about 20 inches or so).
Trifocals are very similar to bifocals. As the name indicates, this style of lens has three sections rather than the two present in bifocals. Like bifocals, trifocals treat presbyopia as well as hyperopia, myopia or astigmatism. Usually, trifocals are suggested for people with advancing presbyopia, where the medium field of vision, between distance and close-up reading, has also become less sharp. The area of the lens that helps this range of vision is called “Intermediate.” It is a thin segment right above the near-vision section of the lens. However, less people are now wearing trifocal lenses due to progressive lenses.
Progressive lenses (also called progressive addition lenses) treat the same conditions as bifocals and trifocals. They are prescribed for people with a combination of presbyopia and hyperopia, myopia or astigmatism. However, the difference is that with progressive lenses the distance and near vision sections of the lens are gradually blended into each other. So there is no border between the sections. Progressive lenses help people with presbyopia to see continuously across the lens, avoiding the jarring jumps that can occur when looking from the near to the distant sections in bifocals. Many people also feel that, in addition to providing better vision, progressive lenses are more attractive than bifocals or trifocals since there isn’t an obvious line at the bottom of the lens.