We often measure the standard of vision achieved with or without glasses using the familiar eye test (Snellen) chart, which is viewed from a distance of 6 metres (20 feet). It should be noted that this is not a particularly accurate way of recording visual standards – other methods are used in laboratory and research settings. The high contrast lettering on a Snellen chart is also unrepresentative of more difficult visual tasks, such as driving at night or in conditions of poor visibility.

The top line on the chart is designated 6/60. The second line up from the bottom (on most charts) is 6/6 and represents ‘normal’ sight – commonly known as 20/20 vision, particularly in the USA, as 6 metres is equivalent to 20 feet. In the UK, the minimum standard of vision permissible for driving is between 6/9 and 6/12, equating to the ability to read a number plate from a distance of 20 metres (20.5 metres for older-style plates). The Royal Air Force requires its pilots to have an unaided vision of 6/5 in each eye (usually the bottom line on the chart).