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One of the main problems with bifocal and trifocal lenses is the issue of eye fatigue. It can be difficult to switch from one focusing power to another. Your eyes can tire, which can even lead to a headache, sore neck and sore back.
A variation of bifocals and trifocals is the no-line lens or progressive lens. No-lines provide a smooth transition from focusing on nearby to distant objects because they do not have a distinct line separating the focusing powers. Instead, a gradual change in power allows the wearer to focus on objects at all distances. Distant objects are viewed through the upper portion of the lens, while near objects are viewed through the middle or lower portion of the lens. These are also great for computer users.
Previous to the last few years, the only materials available for use as lenses were glass and a hard resin called CR-39. But recently, high index lenses have become available. High index materials are named because they have a higher index of light refraction. Basically, they can do the same job that glass or CR-39 does, but high index lenses are much thinner and lighter. With high index lenses, you can avoid having “soda bottle” lenses.
When learning about high index lenses, you may hear many unfamiliar numbers and terms. Here are a few things to remember.
Polycarbonate
The first and still the most popular high index plastic is polycarbonate. Polycarbonate was originally developed for fighter jet cockpits. It is very strong, very light, and resistant to scratches and breaking. Most sports lenses are made of polycarbonate.
Mid-Index
Other high index materials are classified by numbers. The higher the number, the thinner and lighter the lens. The lower numbers are classified as mid-index lenses. Mid-index lenses, such as 1.54, 1.56, and 1.57, are thinner than glass, and nearly as strong as CR-39.
High-Index
High index lense are much thinner than regular glass or plastic. Talk with your doctor to decide which high index lens is right for you.
