The term 20/20 (or 6/6) vision comes from the fact that the eye test chart is intended to be placed at a standard distance away from the patient being measured for visual acuity 20 feet in the United States, and 6 metres in Europe. The first number refers to the distance at which the person being tested can perceive the detail, whereas the second number refers to the distance at which a person with normal vision can perceive the detail. Thus, 20/20 vision means that you can see the same detail 20 feet away as a person with normal vision. 20/40 vision, by contrast, indicates that you can see detail at 20 feet that a person with normal vision can perceive from 40 feet away. The largest letter on the chart – most often the letter E on the Snellen Eye Chart – represents 20/400 (or 6/120) vision. There are letters on the eye chart of a smaller size than that found on the 20/20 line (e.g. 20/12 or 6/4).
In modern day testing rooms where the room length is less than 6 metres, the chart is calibrated for that room distance to simulate the same level of visual acuity.Back