20-20 vision is based on the distance of 20 feet
When you do an eye test, the distance between you and the eye chart is 20 feet. The distance was chosen because the rays of light, as you experience it from your two eyes, are about parallel to an object at that distance. The third line from the bottom on the eye chart is the 20-20 line. If you can see that line fine at 20 feet, you have 20-20 vision. If you have 20-10 vision, for instance, you can spot letters at a distance that others need to stand at 10 feet to see.
The chart that is traditionally used for eye test is the Snellen chart, named after Dutch ophthalmologist Hermann Snellen who developed the chart during 1862. To test a person’s central visual field for blind spots or distortion, the Amsler Grid is used.