According to How Stuff Works, 20/20 vision is the ability to see at 20 feet what a normal person can see at that distance. A person with 20/13 vision is able to see at 20 feet what the average person can see at 13 feet.
How Stuff Works states that 20/20 is the standard for human eyesight. However, as the second number in this fraction increases, so does a person’s visual acuity. For instance, if a person’s measured vision is 20/200, he or she is considered legally blind. However, the stronger a person’s vision is, the lower the second number. A person with vision of 20/10 would have outstanding eyesight.