Emmetropia

Emmetropia (from Greek emmetros, "well-proportioned" or "fitting", + ) describes the state of vision where a faraway 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. A human eye in a state of emmetropia requires no corrective lenses; the vision scores well on a visual acuity test (such as an eye chart test). For example, on a Snellen chart test, emmetropic eyes score at "6/6"(m) or "20/20"(ft) vision, whereas myopic (near-sighted) eyes might score at 20/40 and hyperopic (far-sighted) eyes might score at 20/15.

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. 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.

Accommodation of the lens does not occur in emmetropia. In emmetropia, the lens is about 3.6 mm thick at the center; in accommodation, it thickens to about 4.5 mm. A relatively thin lens and relatively dilated pupil are also associated. The lens usually stiffens with age, causing less ability to focus when the eyes are not in a state of emmetropia.

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