Refractive error basics

Courtesy of wader on Flickr

Courtesy of wader on Flickr

What is a refractive error? With refractive errors, the shape of the eye prevents light from focusing on the retina. Factors include the length of the eyeball (either longer or shorter) or changes in the shape of the cornea or lens.

Myopia, otherwise known as nearsightedness, is a vision condition in which you can see close objects clearly, but objects farther away are blurred. Light is focused in front of the retina.

Hyperopia, or farsightedness, is a vision condition in which distant objects are usually seen clearly, but close objects do not come into proper focus. Light is focused behind the retina.

Astigmatism is a vision condition that causes blurred vision due either to the irregular shape of the cornea, the clear front cover of the eye, or the curvature of the lens inside the eye. The cornea of a normal eye is curved like a basketball. An eye with astigmatism has a cornea that is curved more like a football, with some areas that are steeper or more rounded than others. This may cause images to appear blurry and stretched out. In astigmatism, light is bent differently and is focused at various locations on the retina.

Presbyopia is an age related condition where there is a gradual loss of the eye’s ability to focus on near objects.

Diagnosis of refractive errors can be made by a comprehensive eye examination. Eye examinations are important for all children and adults, starting at 6 months of age. An eye examination will ensure that you can see clearly and comfortably at all distances and detect conditions that may contribute to eyestrain. If needed, glasses, contact lenses or refractive surgery can provide clear and comfortable vision.

~ Melissa Barnett, OD, FAAO
California Optometric Association
http://www.coavision.org
http://www.eyehelp.org

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I have an astigmatism. What is that?

Courtesy of Greece Trip Admin on Flickr

Courtesy of Greece Trip Admin on Flickr

Astigmatism is a term used by optometrists to describe a prescription for one eye that needs two powers to bring it into focus. It is not a disease or something that will make you go blind, but it can make things blurry at distance and at near.
Most people are familiar with the terms “near-sighted” and “far-sighted.” In the eye care world we use the term “myopia” for near-sightedness and “hyperopia” or “hypermetropia” for far-sightedness. These terms are used to describe the power (+ or -) of the lens needed to make you see clearly.
If your prescription needs only one power to bring your eye into focus then you can think of it as being simple. So if your prescription has a number like -4.00, then you have simple myopia. Similarly, if you have a prescription of +2.25, then you have simple hyperopia.
If you have an astigmatism in your eye, then you have two powers that need to be corrected for you to see clearly. Having an astigmatism in your eye is our way of describing a compound prescription. Instead of just one simple power like we described earlier, there are two powers together. Depending on your prescription, you can have myopia with astigmatism or hyperopia with astigmatism.

Courtesy of Ciro Boro - photo on Flickr

Courtesy of Ciro Boro – photo on Flickr

A common example optometrists use to describe astigmatism to patients is the difference between a basketball and a football. A basketball is nice and round, and has only one curve for the entire ball. You can think of that curve as a lens power. A basketball is a good example of a simple prescription. A football, on the other hand, has two curves. This is like an eye that has two powers or an astigmatism.
Astigmatism is not an eye disease, but rather a term we optometrists use to describe a compound prescription in one eye. So don’t be alarmed if your optometrist tells you that you have some astigmatism in your eyes. You are not alone – I have an astigmatism in both of my eyes and I see extremely well!

~Ranjeet S. Bajwa, OD, FAAO
California Optometric Association
http://www.coavision.org