Emmetropia ("em-eh-TROE-pee-ah") is the medical term for naturally occurring clear vision. In other words, a person with emmetropia does not have any refractive errors — such as nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Someone with emmetropia has clear vision at all distances without corrective lenses and without squinting or straining their eyes.

When light enters an emmetropic eye, an image focuses perfectly on the retina. This is unlike nearsightedness, in which light focuses images in front of the retina, or farsightedness, in which light focuses images behind the retina.

In cases of hyperopia, the eyeball is a little too short from front to back; in myopia, the eyeball is slightly too long. Emmetropic eyes are just the right length to allow light to reach the ideal spot on the retina to create crisp, clear vision.

A person with two emmetropic eyes is considered to have “ideal vision” and does not require any sort of vision correction. Emmetropia is often given as the medical definition for 20/20 vision.

What causes emmetropia?

Emmetropia occurs when there is perfect balance between the length and the optical power of the eye. Not much is known about why some people’s eyes develop with this ideal balance while others’ eyes develop a bit long or short. Scientists believe it may involve many factors ranging from genetics to visual input in the environment.

Does emmetropia require treatment?

Because emmetropic eyes naturally produce 20/20 vision, no corrective treatment is needed to aid in visual clarity. However, people who have emmetropia can still develop other eye conditions that should receive proper attention and treatment.