<unk>  is the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state. That condition of the normal eye is achieved when the refractive power of the cornea and eye lens and the axial length of the eye balance out, which focuses rays exactly on the retina, resulting in perfectly sharp distance vision. A human eye in a state of<unk>  requires no corrective lenses for distance; the vision scores well on a visual acuity test (such as an eye chart test). While<unk>  implies an absence of myopia, hyperopia, and other optical aberrations such as astigmatism, a less strict definition requires the spherical equivalent to be between -0.5 and +0.5 D and low enough aberrations such that 20/20 vision is achieved without correction. For example, on a Snellen chart test,<unk>  eyes score at least "6/6"(m) or "20/20"(ft) vision, meaning that at a distance of 20 ft (the first number) they see as well as a "normal" eye at a distance of 20 ft (the second number). Eyes that have enough myopia (near-sighted), hyperopia (far-sighted, excluding latent and facultative hyperopia), or optical aberrations would score worse, e.g. 20/40 (visual acuity of 0.5). Typical<unk>  vision might be 20/15 to 20/10 (visual acuity of 1.3 to 2).<unk> metropes with presbyopia might use lenses for near vision.
Emmetropia
Hypertropia
Anisometropia
Heterotopia 
Presbyopia
Quadrantanopia
Nyctalopia
Pseudomyopia