Visual acuity (VA) commonly refers to the clarity of vision. Visual acuity is dependent on optical and neural factors, i.e.,
(i) the sharpness of the retinal focus within the eye,
(ii) the health and functioning of the retina, and
(iii) the sensitivity of the interpretative faculty of the brain.

A reference value above which visual acuity is considered normal is called 6/6 vision,
the USC equivalent of which is 20/20 vision :
At 6 meters or 20 feet, a human eye with that performance is able to separate contours that are approximately 1.75 mm apart.