When rays are focused correctly on the retina of a relaxed eye, the eye is said to be emmetropic. Emmetropia is the medical term for 20/20 vision, vision that needs no corrective lenses, contact lenses, or reading glasses. It occurs because the optical power of the eye can perfectly focus an image to the retina, giving it “perfect” vision.

The opposite of emmetropia is ametropia. With ametropia, the focal point of the eye is some distance in front of or behind the retina.  The following vision conditions are types of ametropia.