In humans, perfect vision is a visual acuity of 20/20. The first number in 20/20 refers to the distance at which a person can see a particular object in daylight; the second number refers to the distance a human eye with perfect vision can see the object.
While 20/20 is the standard for perfect vision, it is possible for humans to have slightly better vision than this. When the first number is larger than the second number, it describes someone with vision better than 20/20. A person with 20/10 vision sees an object clearly at 20 feet away, while a person with normal vision can only see this object clearly at 10 feet away.
When the first number is smaller than the second number, the person has impaired vision. Someone with 10/20 vision must be 10 feet away from an object that people with normal vision can see from 20 feet away. The threshold for legal blindness in the United States is 20/200.
While visual acuity is used for legal purposes such as determining driving restrictions and setting a standard for blindness, it is not the only medical measurement of visual ability. It does not take into account reductions in peripheral vision, color vision or depth perception.