|Classification and external resources|
An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object 8 meters or further away from the eye. This proves to be an evolutionary advantage by automatically focusing the eye on objects in the distance because it allows an individual to be alert in, say, a prey-predator situation.
An eye that has refractive error when viewing distant objects is said to have ametropia or be ametropic. This eye, when not using accommodation, cannot focus parallel rays of light (light from distant objects) on the retina.
The word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors:
- Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error frequently have blurry vision.
- Myopia: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can easily be corrected with a concave lens which causes the divergence of light rays before they reach the cornea.
- Hyperopia: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). This can be corrected with convex lenses which cause light rays to converge prior to hitting the cornea.
- Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder.
- Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.
- Presbyopia: When the flexibility of the lens declines typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses.
Blurry vision may result from any number of conditions not necessarily related to refractive errors. The diagnosis of a refractive error is usually confirmed by an eye care professional during an eye examination using a large number of lenses of different optical powers, and often a retinoscope (a procedure entitled retinoscopy) to measure objectively in which the patient views a distant spot while the clinician changes the lenses held before the patient's eye and watches the pattern of reflection of a small light shone on the eye. Following that "objective refraction" the clinician typically shows the patient lenses of progressively higher or weaker powers in a process known as subjective refraction. Cycloplegic agents are frequently used to more accurately determine the amount of refractive error, particularly in children 
An automated refractor is an instrument that is sometimes used in place of retinoscopy to objectively estimate a person's refractive error. Shack–Hartmann wavefront sensor and its inverse  can also be used to characterize eye aberrations in a higher level of resolution and accuracy.
Vision defects caused by refractive error can be distinguished from other problems using a pinhole occluder, which will improve vision only in the case of refractive error.
How refractive errors are treated or managed depends upon the amount and severity of the condition. Those who possess mild amounts of refractive error may elect to leave the condition uncorrected, particularly if the patient is asymptomatic. For those who are symptomatic, glasses, contact lenses, refractive surgery, or a combination of the three are typically used.
In the case of myopia, however, some believe that such treatments may also have the long-term effect of exacerbating that refractive error — i.e., making the patient even more nearsighted. This would be due to the very same prescription that is tailored for use at a 12-to-20-foot distance also commonly being used for close-up work as well, thus artificially amplifying the focusing stress that would normally be presented to the accommodation mechanisms of the eye at that distance.
However, this exacerbating effect is not generally believed to exist in the general case, although in cases where the myopia is due to accommodative spasm, removing the corrective lenses for a time may lead to improvement.
The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion.
- Roque, B. Refractive errors in children. November 2, 2005.
- "Frequently Asked Questions: How do you measure refractive errors?". The New York Eye And Ear Infirmary. Retrieved 2006-09-13.
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- "WHO Disease and injury country estimates". World Health Organization. 2009. Retrieved Nov 11, 2009.
- Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. Dubuque: McGraw-Hill, 2010. Print