In a nutshell, there are four types of lenses used to correct vision: Single vision, bifocal, trifocal, and progressive.
Single vision – As the name indicates, a single vision lens has the same focal power (or magnification) from top to bottom and can be used for nearsightedness, farsightedness, as well as for correcting astigmatism. Single vision lenses are the most common lenses used and, for most first time glass wearers, the first lenses prescribed. While you can purchase inexpensive ready-made single vision readers in most drug stores, they will not correct for astigmatism or in the case where your eyes have different corrective needs, i.e. one eye is a +1.50 and the other is a +2.00. Using readers, in that case, can cause an additional eye strain problems.
Bifocal – A bifocal lens has two parts: the separate, very distinct lower portion designed for reading or close-up work and the upper portion containing a prescription for distance or even with no correction at all. Using bifocals allows a person to leave their glasses on at all times while retaining the ability to see at various distances.
Trifocal – Much like the bifocal, the trifocal contains three separate and distinct levels of prescription, correcting for close-up work, a middle portion for intermediate distance, and the top portion for far distance. Trifocals are fairly rare these days with the advent of…
Progressive lenses – Progressive lenses (like bifocals and trifocals) are used for people with multiple corrective needs such as close-up, distance, and middle distance. Unlike bifocals and trifocals, progressive lenses are true multi-focal lenses that allow a seamless progression through all of the lens powers that you need to see, with no lines or sudden jumps from one magnification to another. Thus, your eyes are allowed to focus in a more natural and comfortable manner. Progressive lenses are particularly useful for people who need both a strong power for reading as well as lesser visual assistance for a computer monitor that is slightly beyond comfortable reading distance.
The majority of prescription lenses are made with either CR-39, polycarbonate, Trivex, or, on rare occasions, glass. With higher (or stronger) prescription glasses you may want to consider Hi-Index lenses which, while more expensive, are lenses that are thinner, lighter and aesthetically more attractive by eliminating thick edges.
Glass – Known for it’s optical clarity, glass is used rarely and, thus, is a very expensive alternative for use in prescription glasses. On the plus side, glass is more scratch-resistant than the various plastics and polymers available on the market. On the negative, besides the high cost, glass lenses can be heavy and are not available across the full range of optical needs.
CR-39 – CR-39 is a plastic polymer that has been in use for optical lenses since 1947. CR-39 is lightweight and used more frequently in prescription sunglasses because it absorbs tint more easily than polycarbonate. While CR-39 is the most popular lens material used, it chips more easily and is thicker than polycarbonate, making it unsuitable for rimless or half-rim frames. Using CR-39 lenses for a heavy prescription may lead to the thick “Coke bottle” look in many frames.
Polycarbonate - Polycarbonate lenses are thinner and lighter than traditional plastic eyeglass lenses. They also offer 100 percent ultraviolet (UV) protection (without any additional coatings) and are up to 10 times more impact-resistant than regular plastic lenses. Not only is polycarbonate lighter than CR-39 and glass, unlike them, it will not shatter upon violent impact, making it the safest choice for sports or children’s glasses.
Trivex – Similar to polycarbonate, Trivex is a urethane-based lens which is cast-molded like a plastic-based lens for outstanding optical clarity, while retaining the thin and lightweight qualities (although marginally heavier) than polycarbonate. Trivex is an excellent choice for rimless (or drill-mounted) glasses since it resists the small “spider-cracks” at the drilling points. Trivex also provides 100 percent UV protection.
Lens coatings can provide additional optical benefits as well as improving the appearance of your glasses. Among the coatings available are: Anti-Reflective, scratch coats, and UV (ultraviolet) coatings. Mirror coatings are also available.
Anti-Reflective coatings – Anti-Reflective coatings, also know as ‘AR’ or anti-glare coatings, are coatings generally applied to the front and backside surface of lenses for the purpose of diminishing glare or reflections. If you drive at night a good AR coating shrinks down glare or the star/halo effect from oncoming headlights while improving your night vision. Anti-Reflective coatings have also been proven beneficial when using a computer for extended periods of time by reducing glare off of the monitor, which, in turn, eases eyestrain. From a cosmetic standpoint, AR coatings reduce the shine and reflection off of high index lenses, which tend to be brighter due to their thinness.
Scratch coating – Even metal can be scratched, so what chance does glass or plastic have? While not protective against deep scratches or gouging, a scratch-proof coating can make your lenses more resistant to the fine scratches that come with day-to-day wear and even from cleaning them with something as seemingly soft as paper towels or napkins. Generally a scratchproof coat will come with an extended warranty on your lenses.
UV coating – In much the same way that ultraviolet rays can damage your skin, UV rays are believed to be a contributing cause to cataracts and retinal damage. As noted above, polycarbonate, Trivex, and high index lenses provide natural 100 % UV protection for your eyes. Plastic lenses provide minimal UV protection that can be increased to 100% protection with a clear UV-blocking dye.
Tints – Tinting lenses is an option but is limited to plastic lenses, Trivex, and some hi-index lenses.