Emmetropia
Encyclopedia
Emmetropia describes the state of vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

 where an object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. This condition of the normal eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 is achieved when the refractive power of the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and the axial length of the eye balance out, which focuses rays exactly on the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 resulting in perfect vision. An eye in a state of emmetropia requires no correction.

Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 meters or 20 feet away. The light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort (an emmetropic eye does not need corrective lens
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

es). If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort. In other words, the eye is automatically focused on things in the distance unless a conscious effort is made to focus elsewhere. For a wild animal or human prehistorical ancestors, this arrangement would be adaptive because it allows for alertness to predators or prey at a distance.

Newborns begin hypermetropic and then undergo a myopic shift to become emmetropic.

Etymology

"Emmetropia" is ultimately derived from Greek en/em
Assimilation (linguistics)
Assimilation is a common phonological process by which the sound of the ending of one word blends into the sound of the beginning of the following word. This occurs when the parts of the mouth and vocal cords start to form the beginning sounds of the next word before the last sound has been...

("in") + metron ("measure," not to be confused with metr-/meter "mother") + op[s/t]- ("eye") + -ia ("condition"). Translated literally, the term indicates the condition of an eye's having in itself (i.e., without recourse to corrective lenses or other instruments) the capability to obtain an accurate measurement of an object's physical appearance.

In the press conference following his victory in the 2000 Scripps National Spelling Bee
Scripps National Spelling Bee
The Scripps National Spelling Bee is a highly competitive annual spelling bee in the United States, with participants from other countries as well. It is run on a not-for-profit basis by The E. W...

, George Thampy reported that although upon receiving the word "emmetropia" during the competition he was unsure of its etymology and did not know how to spell it, the voice of God revealed the correct spelling to him in response to his prayer.
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