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Causes
By Mayo Clinic staff
Nearsightedness is a type of refractive error. That means you can't see clearly because the light that enters your eye doesn't bend (refract) properly.
Normal vision
To focus the images it sees, your eye relies on two critical parts:
- The cornea, the clear front surface of your eye
- The lens, a clear structure inside your eye that changes shape to help focus objects
In a perfectly shaped eye, each of these focusing elements has a perfectly smooth curvature like the surface of a rubber ball. A cornea and lens with such curvature bend (refract) all incoming light in such a way as to make a sharply focused image on the retina, at the back of your eye.
A refractive error
However, if your cornea or lens isn't evenly and smoothly curved, light rays aren't refracted properly, and you have a refractive error. Nearsightedness is one type of refractive error. Nearsightedness can occur when your cornea is curved too much or when your eye is longer than normal. Instead of being focused precisely on your retina, light is focused in front of your retina, resulting in a blurry appearance for distant objects.
Other refractive errors
In addition to nearsightedness, other refractive errors include:
- Farsightedness (hyperopia). This occurs when your cornea is curved too little or your eye is shorter from front to back than normal. The effect is the opposite of nearsightedness. Light is focused beyond the back of your eye, making nearby objects blurry. You're usually able to see faraway objects clearly.
- Astigmatism. This occurs when your cornea is curved more steeply in one direction than in another. Uncorrected astigmatism blurs your vision. Typically, the images you see will be blurred more in one direction than another. For example, horizontal images may be more out of focus than are vertical or diagonal images.
- Care of the patient with myopia. St. Louis, Mo.: American Optometric Association. http://www.aoa.org/documents/CPG-15.pdf. Accessed Oct. 14, 2009.
- Preferred practice patterns: Refractive errors and refractive surgery. American Academy of Ophthalmology. http://one.aao.org/CE/PracticeGuidelines/PPP_Content.aspx?cid=e6930284-2c41-48d5-afd2-631dec586286. Accessed Oct. 14, 2009.
- Refractive error. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/print/sec09/ch099/ch099a.html. Accessed Oct. 13, 2009.
- Frequency of ocular examinations. American Academy of Ophthalmology. http://one.aao.org/CE/PracticeGuidelines/ClinicalStatements_Content.aspx?cid=810eaf61-181e-41c8-a0e8-e1d122efe5a4. Accessed Oct. 14, 2009.
- Opticians, dispensing. U.S. Department of Labor. http://www.bls.gov/oco/ocos098.htm. Accessed Oct. 15, 2009.
- Bower KS. Laser refractive surgery. http://www.uptodate.com/home/index.html. Accessed Oct. 14, 2009.
- Eye health tips. National Eye Institute. http://www.nei.nih.gov/healthyeyes/eyehealthtips.asp. Accessed Oct. 15, 2009.
- Robertson DM (expert opinion). Mayo Clinic, Rochester, Minn. Oct. 20, 2009.
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