Emmetropia (from Greek emmetros, "well-proportioned" or "fitting", + -opia) describes the state of vision where a faraway object at infinity is in sharp focus with the crystalline lens in a neutral or relaxed state.

Emmetropia does not require visual correction (most commonly spectacles and contact lenses) should not be confused with ametropia, which results in blurred uncorrected vision.

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. 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 thus requiring optical intervention to restore focus and clear vision.

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.

The development of an eye towards emmetropia is known as emmetropization. This process is guided by visual input, and the mechanisms that coordinate this process are not fully understood. It is assumed that emmetropization occurs via an active mechanism by which defocus drives growth of the eye and that genetic factors and emmetropization both influence the growth of the eye's axis.

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

Ametropia is the collective name for any deviation in the relaxed focus of the eye from the normal state in which distant objects form sharp and clear images on the retina. If you are not emmetropic, then you have a refractive error and referred to as ametropic.

Examples of refractive error are the following:

Myopia or short-sightedness

Hyperopia or far-sightedness

Astigmatism

Presbyopia

All of which requires optical intervention, most commonly spectacles and contact lenses to restore clear vision.

There has been some research on causal factors involved in the development of ametropia. In particular, statistics show that prolonged near work correlates with the development of myopia, but it is still unclear whether there is a causal relation.Furthermore, outdoor activity has been found to have a protective effect on myopia development in children.