Description: This guide reveals the various lenses available and how to read an eyeglass prescription.
How to Choose the Right Lens for Your Glasses
The lens you choose for your glasses will determine how comfortable and helpful your eyeglasses are going to be. Today there are as many lenses as there are frame designs so choosing one can be difficult. The following information will explain the different types of lens available, and knowing what’s out there is going to help you make the right decision. The Benefits of Choosing the Right Lens Your choice should depend on these factors: safety, vision, comfort and appearance. Design, coatings and materials have to be considered as well. Bear in mind there are many different types of prescription glasses, with single vision lenses for astigmatism, farsightedness or nearsightedness. There are also progressive lenses, multifocal lenses for presbyopia and bifocals.
Materials for eyeglass lens consult your eye doctor for other options and what’s best for you.
- Glass lenses: the earliest lenses were made of glass, and these are still available. They offer superior optics but break easily and therefore can pose a serious threat to your eyes.
- Plastic lenses: lightweight plastic eyeglass lenses first came out during the late 1940s and were constructed from CR-39 plastic polymer. CR-39 weighs only half as much as glass, cheap and offers good optical qualities so it’s still popular today.
- Polycarbonate lenses: during the 1970s, the Gentex Corporation began offering polycarbonate lenses for use on glasses. These glasses found wide acceptance and by the 1980s had become widely used, and still used today. These glasses were originally designed for use on helmet visors for the United States Air Force, for bulletproof glass and other applications. However, its reliability, safety and dependability made it ideal for use as eyewear, especially for children. It is lighter than CR-39 and has greater impact resistance, making it ideal not just for children’s eyewear but also for sports eyewear and safety glasses.
- Trivex: introduced by PPG Industries in 2001, this is being touted as an alternative to polycarbonate as it is lightweight and with impact resistant features.
- High-index plastic lenses: the past two decades has seen an increase in the demand for lighter and thinner eyeglasses, and this has led to the development of several high index lenses. Many of them are lighter, stronger and thinner than CR-39 because of their higher refraction index, which makes them attractive to many people.
What is the Index Refraction?
The refraction index, or refractive index of your lens material is a measure of how effective material bends, or refracts light, which is dependent on how fast light can travel in the material. To be more specific, the lens material’s refractive index is the speed of light’s ratio in a vacuum, divided by speed of light in the lens. For example, if the CR-39 plastic has an index refraction of 1.498, it would mean that light moves 50% slower in the CR-39 plastic compared to that of a vacuum.
The higher the refractive index of a material is, the slower light travels and leads to more bending / refracting of the light. The higher the refractive index, the less lens material is necessary to refract the light. The refractive index for lens materials today are from 1.498 (CR-39 plastic) to 1.74 (high index plastic). What this means is that lens constructed from CR-39 plastic is going to be the thickest available while the 1.74 lens will be the thinnest.
How to Read an Eyeglass Prescription
When you get an eyeglass prescription you’re going to see the words OS and OD with numbers under them. OS stands for oculus sinister, which is Latin for your left eye, and oculus dextrus for your right eye. Sometimes you will see OU instead, which means both eyes. To make it simple, the further the numbers are from zero, the more severe your eyesight condition is and the more correction is necessary. If there’s a plus sign on the number, it signifies that you are farsighted, and if there is a minus sign it means you are nearsighted. The numbers on your prescription are indicative of diopters, which is the unit used for measuring the focusing power or correction of the lens needed by your eyes. In prescriptions, diopter is usually shown as “D”.
Suppose your prescription reads -1.00: it means nearsightedness of 1 diopter, and it’s just mild. If the prescription says -4.25, that signifies your nearsightedness level is 4 and 1/4 diopters. This also means your nearsightedness is greater compared to -1.00 and therefore requires thicker lenses. The same is true if your prescription says +1.00, as that is just a bit of farsightedness whereas +5 is greater. If you are astigmatic, there will be three numbers and they will usually be written as S x C x Axis. The S here is in reference to the “spherical” part of your prescription, and is the degree of your farsightedness or nearsightedness as indicated above. The C on the other hand, refers to the astigmatism or “cylinder”. This can be negative or positive and is used to measure your astigmatism, also in diopters. The higher the number, the more astigmatic you are.
The axis mentioned in your prescription is going to be a number from 0 to 180 degrees and tells you the astigmatism’s orientation. A typical prescription for astigmatism will look like this:
- -2.00 +1.50 x 180
- +3.50 +3.00 x 45
In the first prescription, you have a nearsightedness of 2 diopters 1.5 diopters of astigmatism and 180 axis degrees. In the second prescription, you have .5 diopters of farsightedness, 45 degree axis and 3 diopters of astigmatism. These are just examples of course, but you should get a general idea of what they are and how they work.
Keep in mind that prescription for eyeglasses and contact lens are not the same, and if you’ve been given prescription for eyeglasses, you can only use that for glasses and not for contact lenses.. If you want those, a separate prescription and consultation will be necessary. This is very important and must not be confused.