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GUIDELINES FOR CHOOSING THE RIGHT SIZE FRAME FOR YOUR PRESCRIPTION
There are usually two components to any eyewear prescription.
You are either nearsighted, which means you can see at near but not at a distance, or farsighted, which means opposite.
About 70% of eyeglass wearers also have some form of astigmatism. This simply means that front part of eye, cornea, has 2 different curvatures (like a football). So lenses are designed with two different curves to compensate for shape of eye to correct astigmatism.
Here's an example of an eyeglass prescription:
OD -3.00/-1.50 X 180 OS 2.50/-1.00 X 165
This is what it means.
OD is a latin abbreviation for right eye. OS is a latin abbreviation for left eye.
The first number (-3.00 in example) is called sphere. This tells optician what power to make lens to correct nearsightedness (-) or farsightedness ( ). In our example right eye (OD) is nearsighted and left eye (OS) is farsighted.
The numbers after (/) refer to amount of astigmatism. The (X) is an abbreviation for word 'axis' and numbers 180 and 165 indicate placement in degrees of astigmatic lens.
THE NUMBERS THAT CONCERN YOU WHEN DECIDING THE BEST SIZE FRAME FOR YOUR PRESCRIPTION ARE THE SPHERE NUMBERS (-3.00 and 2.50 in our example).
If your prescription is less than -2.50 or 2.50 almost any size and shape frame will be suitable for your prescription. You do not require hi-index or thin and light prescription lenses. Any frame with regular plastic lenses will have an acceptable edge thickness and should look cosmetically pleasing to you.
For prescriptions of -2.50 to -4.00 you should choose a frame with an eye size of 54 or less. The size is written on inside arm of frame. If you have to have a frame with a larger size because of style or face shape, then make sure you get hi-index or thin and light lens. Otherwise, your lenses will have thick edges.
Do not choose a rimless frame if your prescription is over -2.50 unless you use a high index lens, or edge thickness of lens will be a problem.
For prescriptions of -4.00 to -6.00 it is advisable to order your lenses in a hi-index material and to keep frame size as small as possible. This will ensure that your glasses look thinnest.
If you are unsure about which lenses are best suited for your prescription, or if you have a very difficult or high prescription call 1-800-248-9427 and ask for Lens Information and Assistance.
Trained personnel will help you understand your prescription and which lenses you should use. The service is FREE.
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UNDERSTANDING SINGLE VISION LENS MATERIALS
There are many single vision lens materials, but most commonly used polymer is a plastic material known as CR-39. Because of its light weight, it is very comfortable and can be tinted almost any color and density.
However, certain manufacturers of CR-39 lenses produce a low quality and inferior product. Often, lenses will be warped, causing 'soft spots' of poor vision throughout lenses.
Manufacturers that create exceptionally high optical quality lenses, all of which can be ordered with a very effective scratch protection coating, are SEIKO, SILOR and SOLA.
Thin-plastic (hi-index) and light lenses are also available. The refractive index of this material is higher than regular plastic lenses. This means that a thinner lens can do same job that a thicker lens would normally do. These lenses are about 35% thinner and lighter than regular plastic lenses.
Be aware, however, there are different grades and qualities of these lenses. The higher refractive index, thinner lens.
The best quality hi-index lenses have a refractive index of 1.60 or 1.66. Some opticals still use lenses with a 1.54 refractive index, but charge price of a 1.60 lens, because John Q. Public doesn't know difference. If your prescription requires hi-index lenses, insist on 1.60 or 1.66 refractive hi-index lenses for best quality and thinnest lenses.
We recommend following hi-index lenses:
- Silor Thin & Lite 1.60 refractive index - Pentax THC 1.60 refractive index - Pentax 1.66 Ultrathin with anti-reflective - Seiko Super 16 MX - Seiko Super 16 diacoat - Optima Aspheric 1.66 - Optima Aspheric 1.60 - Optima Hyper 1.60 - Any Sola product
Another material, called Polycarbonate, is often marketed as a thin and light lens. It is softer than CR-39, may scratch and can't be tinted as dark as plastic lenses. It definitely is lighter and thinner than regular plastic. Many opticals promote this material over hi-index because it costs less, yet they often charge same price as hi-index lenses.
Polycarbonate is extremely resistant to shattering, so it is recommended for children or persons needing safety eye protection. However, newerhi-index lenses sometimes have superior optics compared to polycarbonate which occassionally has some peripheral distortion. If you are being sold a thinner and lighter lens, ask if it is polycarbonate or hi-index plastic.
REMINDER: POLYCARBONATE AND HI-INDEX PLASTIC LENSES AUTOMATICALLY COME WITH SCRATCH PROTECTION AND ULTRAVIOLET PROTECTION. IF YOU PURCHASE EITHER ONE OF THESE TWO MATERIALS DO NOT BE SOLD THESE COATINGS AS 'ADD-ONS.'
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Dr. David Cooper and Dr. Guy Hodgson are the founders of FramesDirect.com which has been recognized for outstanding customer service and satisfaction. FramesDirect.com is fast becoming the global leader of prescription eyeglasses, contact lenses and sun glasses sales online.