In optometry, 20/15 vision is considered to be above average in terms of eye sight. By most metrics, 20/15 is the best vision rating that a person can receive and is even better than 20/20 vision.
20/20 vision represents what is thought of as average vision. A person with 20/15 vision, however, is expected to be able to read a line of text on an eye chart 5 feet closer than someone with average 20/20 vision. This explains why the second number is 15 instead of 20. In the United States, less than 10 percent of the population has been diagnosed with 20/15 vision.