Visual acuity refers to the ability to discern fine visual detail. The distance for testing the eye is 6 metres or 20 feet.At this distance the normal eye is at rest.The procedure for measuring the distance vision is done with a Snellen chart. It is specific-sized letters at a routinely-fixed distance.

The smallest "Snellen-letters" is called "20/20", because you see at 20 feet, what a normal eye should see at 20 feet. Legal blindness is 20/200 vision. This means you can only recognize something 20 feet away that a normal eye is able to see 200 feet away. Visual acuity of 20/25 or even 20/30 may be acceptable for some patients but, generally, patients with below-normal visual-acuity ( 20/30 or worse ) due to refractive error will require optical correction or, if that is not possible, some other form of visual assistance.