For fighter pilots, it is a minimum requirement. But having 20/20 ‘visual acuity’ (correct term) does not necessarily mean you have perfect vision (as convention assumes); instead, it indicates sharpness and clarity of vision at a distance. It is measured by a Snellen Chart, which displays letters of progressively smaller size and whereby 20/20 means that the test subject sees the same line of letters at 20 feet that a person with normal vision sees at 20 feet (or 6 metres; but 6/6 simply didn’t catch on).