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Emmetropia is the term used to describe a person’s vision when absolutely no refractive error or de-focus exists. Emmetropia refers to an eye that has no visual defects. Images formed on an emmetropic eye are perfectly focused, clear and precise. When an eye is emmetropic, light rays coming into the eye from a distance come to perfect focus on the retina.
Nearsightedness (myopia) is a common vision condition in which you can see objects near to you clearly, but objects farther away are blurry. It occurs when the shape of your eye causes light rays to bend (refract) incorrectly, focusing images in front of your retina instead of on your retina.
Nearsightedness may develop gradually or rapidly, often worsening during childhood and adolescence. Nearsightedness tends to run in families.
Farsightedness (hyperopia) is a common vision condition in which you can see distant objects clearly, but objects nearby may be blurry.
The degree of your farsightedness influences your focusing ability. People with severe farsightedness may be able to clearly see only objects a great distance away, while those with mild farsightedness may be able to clearly see objects that are closer.
Astigmatism is a blurring of vision due to an irregular shape in the cornea or lens. In eyes without astigmatism, the cornea and lens have a more or less similar curvature in all directions. This allows light to be focused to a single point on the retina.
People with astigmatism have more curvature in one direction, or meridian, than in another so that light is not able to focus on a single point on the retina. This results in blurring of vision at all distances.