The farther the numbers are from 0, the worse is your eyesight, and the more vision correction you will need. The numbers are in. Someone with 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. 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 degrees. It refers to the orientation of your astigmatism. Two examples of a prescription with astigmatism are:.
Person One: Person One has 2 diopers of nearsightedness, with 1. Person Two has 3. 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 person doing your eye exam will switch between two different lenses and say, "Do you like this one or that one? There is no right answer — just what is best for you. Remember, the idea is that you get the correct prescription so your vision will be top-notch!
If you're going to get glasses, it's time to pick frames. It can be fun to try these on. Choose ones that are comfortable and sturdy. But also make sure you like them — you'll be wearing them a lot! The lenses themselves can be made of different materials, such as safety glass or shatterproof plastic polycarbonate. Because glass tends to be heavy and it may shatter, most glasses today are made of polycarbonate plastic.
If you play sports, you may want to ask about special eyewear you can wear on the field. With glasses, you'll also want to find out how to clean them properly. And it helps if you have a glasses case and put them in it when you're not wearing them. The last thing you want is to sit on your new glasses. If you're going to get contact lenses, you'll get some advice from the ophthalmologist or optometrist about which kind will be best for you. Some are disposable and others need to be cleaned.
When you learn which type you're going to get, you can start becoming an expert in how to wear them safely and keeping them clean. The most important thing about contact lenses is good hygiene to prevent infections in your eye. But the really fun part of new glasses or contact lenses is how well you can see.
They can make your whole world look better! Reviewed by: Jonathan H. Salvin, MD. Larger text size Large text size Regular text size. Why do some people need glasses and others don't? How Eyes Work The eyeball includes: the cornea say: KOR-nee-uh , clear tissue that helps the eye focus the iris , the colored part the pupil the black circle in the center of the iris, which is really an opening in the iris, that lets light enter the eye the lens , which also helps the eye focus the retina , at the very back of the eye.
Advances in automation are rapidly changing how lenses are made. For example, the vast majority of labs now use computers to determine curve parameters and lens choice, and equipment is available that will combine several steps or even do the entire operation automatically. For more information about corrective lenses and related topics, check out the links on the next page.
Bob Broten is an American Board of Opticianry-certified optician and certified laboratory technician at Lenscrafters Inc. He holds a bachelor's degree in biology and did extensive research in fish vision while pursuing his degree.
Author's note: I am indebted to Erik Schopp, A. O-certified optician and general manager of Lenscrafters , and Dr. Dawne R. Griffith, O. Robert D. Optics and optometry are complex subjects beyond the scope of this article. In presenting the basic principles of these two disciplines, I've oversimplified somewhat for the sake of brevity.
For this I apologize. Any errors in fact or theory are entirely mine. I encourage interested readers to seek professional advice, as this article is a brief overview and not intended as a guide to diagnoses.
Also, I am grateful to Lenscrafters store in Portland and to Joshua Boyd, lens technician, for help in taking the photos used with this article. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Everyday Innovations. How Corrective Lenses Work. Test your eyes. How Your Eye Focuses On the back of your eye is a complex layer of cells known as the retina.
When you look at something, three things must happen: Advertisement. The image must be reduced in size to fit onto the retina. The scattered light must come together -- that is, it must focus -- at the surface of the retina. The image must be curved to match the curve of the retina. Read More. Out of Focus Sometimes, for different reasons, the eye doesn't focus quite right: The surfaces of the lens or cornea may not be smooth, causing an aberration that results in a streak of distortion called astigmatism.
The lens may not be able to change its curve to properly match the image called accommodation. The cornea may not be shaped properly, resulting in blurred vision. Myopia nearsightedness occurs when a distant object looks blurred because the image comes into focus before it reaches the retina.
Myopia can be corrected with a minus lens, which moves the focus farther back. Hyperopia farsightedness occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina.
Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing. Astigmatism is caused by a distortion that results in a second focal point.
It can be corrected with a cylinder curve. How a Lens Works " ". Determining Lens Strength The strength of a lens is determined by the lens material and the angle of the curve that is ground into the lens.
A spherical lens looks like a basketball cut in half. The curve is the same all over the surface of the lens. A cylindrical lens looks like a pipe cut lengthwise.
The direction of a cylinder curve's spine axis defines its orientation. It will only bend light along that axis. Cylinder curves are commonly used to correct astigmatism, as the axis can be made to match the axis of the aberration on the cornea. Compound lens: a lens having both a spherical and a cylindrical component Cylindrical curve: a curve that radiates along a straight line, like a pipe cut lengthwise Diopter D : the refractive power of a lens; the higher the number, the stronger the lens Refraction: the bending of light Spherical curve: a curve that is the same in all directions, like a basketball cut in half.
Lens and Prescription To make a lens, the first thing you need is a lens blank. How to Read the Prescription Most prescriptions have four parts: Advertisement. The base spherical strength and type plus or minus The cylinder strength and type The cylinder axis orientation in degrees with 90 degree vertical; an "x" means "at" The strength of bifocal segment "plus" indicating "in addition" and type.
Base curve: a simple spherical curve; the primary lens curve Lens blank: basic spherical lens; the lab grinds the back of the blank to match the prescription Optical center: a spot on a spherical lens where light enters at a degree angle to the lens plane Segment: the portion of a lens added for reading bifocal or trifocal ; it may be added separately to the lens blank or formed as a blended curve onto the base.
Overview: How the Lens is Made " ". The total power in diopters the finished lens must have. The strength and size of the segment if needed. The power and orientation of any cylinder curves.
Details such as the location of the optical center and any induced prism that may be needed. In most labs the equipment is designed to grind minus curves, so a strong, plus lens blank is usually selected.
If the base curve is too strong, then a minus curve is ground in the back of the lens, which reduces the total power of the lens.
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