20/20 vision is actually an American term used to express normal distance visual acuity (acuity is the clarity or sharpness of your vision). It uses black letters on a white background measured at a distance of 20 feet. If you have 20/20 vision, you can see the line of letters at 20 feet that a ‘normal’ eye can see at that distance. If your vision is only half as good as that of a normally-sighted person, you would only be able to see at 20 feet what they could see at 40 feet and that would be recorded as 20/40 vision. The lines of letters on an eye test chart go smaller than 20/20 because some people are able to see even smaller detail than is considered ‘normal’.
However we are in the UK, not the USA so we work in metres not feet! So, actually we call it 6/6 vision as we have our eye charts at 6 metres (3 meters from your eye to the mirror at the end of the consulting room and 3 metres back) instead of the USA’s 20 feet. Increasingly other methods of ‘scoring’ and recording vision are used – either using a digital formal (whereby 6/6 would be recorded as 1.0) or LogMar where each letter read has a number value and these are added or subtracted to give a score. In LogMar 20/20 (or 6/6!) vision would be recorded as 0.0.
For people who cannot recognise and name the letters on the chart we have symbols, pictures or matching letters which can be used to ensure nobody is prevented from getting the best from our eye exmainations. Modern eye test charts are computerised so that letters and symbols can be randomised to stop you from memorising them. They can also be calibrated for exactly the correct distance if the examination room is not precisely 3 metres long.
Having 20/20 vision does not necessarily mean you have perfect vision. 20/20 vision only indicates one aspect of visual performance - the sharpness or clarity of high contrast (black on white) vision at a distance. Other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and colour vision, contribute to your overall visual ability. During your eye examination a combination of tests and observances are used to give an overall picture of your overall visual ability, not just your visual acuity and ensure we can help you get the best possible vision for your needs.