How Do Eyeglass Lenses Work to Correct Your Vision?
You just got your new eyeglass prescription from your doctor and its an impossible-to-understand mishmash of numbers and letters. What does it mean?
We're here to decode it for you. First, we've got to learn a little about how the human eye works.
The outermost portion of your eye is the clear cornea. Behind it is, the iris, the colored portion of your eye. In the center of the iris is the pupil, through which light enters your eye.
Just behind the pupil is the lens, whose job it is to focus the light entering your eye. That light falls onto the retina at the back of your eye, which is a complex layer of cells that react to light. The optic nerve carries those signals to your brain, where they are decoded into an image.
Left is "Sinister"
First, your prescription will say OD and OS. These are abreviations for the Latin oculus dextrus, the right eye, and oculus sinister, the left eye. Sometimes, your prescription will also contain OU, for oculus uterque, which means both eyes.
Next, your prescription will contain either a plus sign ( + ) or a minus sign ( - ) in front of some numbers. Where "0" is perfect 20/20 vision, a plus sign means you are farsighted, and a minus sign means you are nearsighted.
Farsighted people see clearly at a distance, but things up close are blurry. Nearsighted people see things clearly near, but things at a distance are blurry.
Next to either a plus or minus sign are numbers. These are diopters, and they are the unit of measure for the correction, or focusing power of the lenses your eyes require. Diopter is often abbreviated with a "D".
The farther the numbers are from 0, the worse is your eyesight, and the more vision correction you will need. The numbers are in .25 increments. Someone with -1.00 on their prescription is only mildly nearsighted, while someone with -4.50 is more nearsighted.
A person with +1.25 would be mildly farsighted, while someone with +5.25 would be more farsighted.
A Football Rather Than a Basketball
Astigmatism arises out of the cornea being shaped more like a football than a basketball.
If you have astigmatism, your prescription will be written in the form:
S x C x Axis, where "S" is the "spherical", or power, portion of your prescription, which we described above. "C" refers to "cylinder", which is a measure in diopters of how much astigmatism you have.
It can be a negative or a positive number, and the higher the number, the more astigmatism you have.
Axis is a measure in degrees, and will be between 0 and 180 degrees. It refers to the orientation of your astigmatism. Two examples of a prescription with astigmatism are:
Person One: -2.00 + 1.50 x 112
Person Two: +3.50 +3.00 x 58
Person One has 2 diopers of nearsightedness, with 1.5 diopters of astigmatism at an axis of 112 degrees.
Person Two has 3.5 diopters of farsightedness, and 3 diopters of astigmatism at an axis of 58 degrees.
Correcting Your Vision
Now that you know what your prescription means, how will your eyeglasses go about correcting it?
If you are nearsighted, the focal point of the light rays entering your eye will fall in front of the retina. If you are farsighted, the focal point of the light rays entering your eye will fall in back of the retina. If you have astigmatism, there will be multiple focal points, and the image will be blurry.
Eyeglass lenses attempt to correct these problems, and place the focal point where it needs to be — on your retina.
A lens is just two prisms put together back to back. A prism is always thicker at one end and thinner at the other end. Light passing through a prism is bent, or refracted, toward the thicker end.
There are several types of lenses. Convex lenses bend light inward, and they correct farsightedness. The stronger the convex lens, the closer the focal point is to the lens.
Concave lenses bend light outward, and are used to correct nearsightedness. The stronger the concave lens, the farther the focal point is from the lens.
Astigmatism is corrected with a cylindrical lens, where one side of the lens is concave, and the other side is flat. It focuses incoming light into a line.
If you need both a power correction and correction for astigmatism, you will need a compound lens. It combines a spherical and a cylindrical component, and the strength of the lens is the algebraic sum of the two lens types: +1.00 lens + -4.00 lens = -3.00 lens
The Curse of Old Age
Sometime in your 40s, you will experience presbyopia. This where the lens of your eye loses some of its elasticity, and thus its ability to focus light from near objects. People experience presbyopia by having to hold printed material, such as a menu in a restaurant, or their cell phone, farther away.
Today, to correct presbyopia, there are progressive lenses, where the correction changes continuously along the lens. But, a lot of people still use the previous solution for presbyopia — bifocals.
Like their name suggests, biofocals have two focal distances — one for near and the other for far. Since we typically look down when we focus on something near, the near focal correction is at the bottom of the lens.
If your doctor writes you a prescription for a bifocal lens, it will look something like this: +2.50 -1.50 x 119 plus +2.00, where the "plus +2.00" is for the bifocal.
Your eyeglass lenses can be made in any one of four materials:
|Feature||Plastic||Polycarbonate||Hi-Index 1.67||Hi-Index 1.74|
|Prescription Range||Up to +/- 1.75D||Up to +/- 4.00D||Up to +/- 9.00D||Extra-strong prescriptions|
|Thickness||Basic||30% thinner than basic||45% thinner than basic||50% thinner than basic|
Plastic lenses are budget-friendly. Polycarbonate lenses are best for children, athletes, people with mild to moderate prescriptions, and for those with rimless or semi-rimless eyeglass frames.
Hi-Index 1.67 lenses are slimmer and more lightweight than polycarbonate lenses, so you can choose from a wider selection of frames. Hi-Index 174 are the thinnest and lightest lenses, and they provide the highest visual clarity.
Now that you know how to decode your eyeglass prescription and how eyeglasses work, you can take advantage of one of the companies that allow you to order your glasses online, and maybe save on your next pair of eyeglasses.
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