Emmetropia
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Emmetropia (from the Greek emmetros, "well-proportioned, fitting") describes the state of 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 is achieved when the refractive power of the cornea and the axial length of the eye balance out, which focuses rays exactly on the retina 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 lenses). 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.[1]
[edit] Notes
[edit] References
- Bernard Gilmartin, 1998. Myopia and nearwork Pg 33-34, Elsevier Health Sciences. ISBN 0750637846
- David de Angelis, 2008.The Secret of Perfect Vision: How You Can Prevent and Reverse Nearsightedness Pg 33, North Atlantic Books. ISBN 1556436777
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