Emmetropia and Refractive Errors

Refraction with a manual phoropter. Zero Creative

Definition: 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.

Eyes that have emmetropia do not require vision correction. When a person has emmetropia in both eyes, the person is described as having ideal vision.

When an eye is emmetropic, light rays coming into the eye from a distance come to perfect focus on the retina.

If the eye is an abnormal length or the cornea is abnormally shaped, chances are you will not be emmetropic. If a person’s eye is longer than average, light may be focused in front of the retina instead of directly on it. This can cause nearsightedness. If a person’s eyeball is too short, the images are focused behind the retina. This causes farsightedness.

The general condition of emmetropia or how the eye develops toward emmetropization interests scientists and eye doctors. Knowledge of how a growing infant develops toward emmetropization will hopefully show them new ways to correct refractive errors, such as nearsightedness and farsightedness or help find ways to prevent an increase in refractive errors. The process of emmetropization is not well understood but it is thought to occur by visual input, brain activity, genetics and other mechanism where defocus can affect growth of the eye.

If you are not emmetropic, then you have a refractive error. Examples of refractive error are the following.

  • Nearsightedness: Nearsightedness, or myopia, is a condition in which nearby objects are seen clearly, but distant ones are blurred. Nearsightedness can be inherited and is often discovered during childhood. However, you can develop nearsightedness in early adulthood. People that develop myopia in early adulthood usually do not develop high amounts of nearsightedness.
  • Farsightedness: Farsightedness, or hyperopia (also referred to as hypermetropia), usually causes distant objects to be seen clearly, but close objects to appear blurred. Farsightedness often runs in families. When someone has higher levels of farsightedness, their distance vision may become blurry in addition to their near vision. Many people mistake farsightedness for presbyopia, the refractive error that usually occurs over 40 years of age.
  • Astigmatism: Astigmatism usually occurs when the cornea has an irregular curvature. The cornea is curved more in one direction, causing blurry vision. Astigmatism can cause blurry vision at all distances, and it often occurs along with farsightedness or nearsightedness. Most people have very small amounts of astigmatism. Larger amounts of astigmatism cause distortion in addition to blurry vision. Very high amounts of astigmatism sometimes have a difficult time achieving 20/20 vision.
  • Presbyopia: Presbyopia is the normal aging process of the lens of the eye. It is the loss of elasticity of the lens that occurs with aging, causing difficulty focusing at close ranges. Scientists also believe that in addition to the loss of elasticity of the lens, the muscle that makes the lens change focus, called the ciliary body, also begins to not work as well. Presbyopia usually becomes significant after the age of 40-45 years of age but people between 35-40 may exhibit early signs depending on their visual state, work, and lifestyle

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