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Causes
By Mayo Clinic staff
If you're nearsighted, the light rays that enter each eye are focused in front of the retina, instead of on the retina. This causes blurry images.
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 crystalline lens, a clear structure inside your eye that changes shape to help focus objects
In a normally shaped eye, each of these focusing elements has a perfectly smooth curvature like the surface of a smooth 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, more commonly, 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 of 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. When the eye is in a relaxed state, light will be focused beyond the back of your eye, making objects blurry. With a little effort, the eye can focus on distant objects making them clear. With greater effort, the eye can focus on near objects to allow them to be seen clearly. Problems with blurring occur when the crystalline lens begins to age and it loses its flexibility and focusing ability. 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.
- 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 Dec. 20, 2011.
- Facts about myopia. National Eye Institute. http://www.nei.nih.gov/health/errors/myopia.asp#7. Accessed Dec. 21, 2011.
- Myopia (nearsightedness) American Optometric Association. http://www.aoa.org/myopia.xml. Accessed Dec. 20, 2011.
- Refractive error. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merckmanuals.com/professional/eye_disorders/refractive_error/overview_of_refractive_error.html. Accessed Dec. 20, 2011.
- Frequency of ocular examinations. American Academy of Ophthalmology. http://one.aao.org/CE/PracticeGuidelines/ClinicalStatements_Content.aspx?cid=810eaf61-181e-41c8-a0e8-e1d122efe5a4. Accessed Dec. 20, 2011.
- Mian SI. Visual impairment in adults: Refractive disorders and presbyopia. http://www.uptodate.com/home/index.html. Accessed Dec. 21, 2011.
- Opticians, dispensing. U.S. Department of Labor. http://www.bls.gov/oco/ocos098.htm. Accessed Dec. 20, 2011.
- Eye health tips. National Eye Institute. http://www.nei.nih.gov/healthyeyes/eyehealthtips.asp. Accessed Dec. 21, 2011.
- Robertson DM (expert opinion). Mayo Clinic, Rochester, Minn. Dec. 29, 2011.
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