20/20 vision has become a commonly used and simple term to describe how well somebody can read letters on an eye test chart compared with most of the population. In 1862 Hermann Snellen, a Dutch Ophthalmologist, designed a standard chart that could be used to measure and compare the visual acuity of his patients.
This chart is the basis of most of the test charts used by Opticians, Optometrists and eye departments in the world today and is intended to be used at a fixed distance – 20 feet (or 6 metres) from the patient. Why 20 feet? This is the distance at which the level of vision measured represents infinity and this is the first or topmost number in the 20/20 fraction. The lower number indicates the smallest size of letter the patient can make out on the chart. So, if you can read the 20/20 line you are said to have ‘normal’ visual acuity because that is the level that most people can attain in a normal population. If the best you can achieve is the 20/40 line then you see at 20 feet what a ‘normal’ eye would see at 40 feet. The largest letter on the chart is usually 20/200. Obviously the smaller the lower number the better your visual acuity. In Europe we have adopted metres as a test distance instead of feet and 6 metres is conveniently close to 20 feet.
One very important point to understand is the idea of ‘normal’. If your visual acuity is 20/40 and cannot be improved with spectacles, contact lenses or surgery does it mean you have abnormal vision or an unhealthy eye? Absolutely not true! Your eye may be perfectly healthy (i.e. no disease) but only manage 20/40 or 20/50 because, for example, the lens inside has an unusual shape or the pupil is large or because the information processing in your brain has developed differently to other people. Furthermore many patients can see better than 20/20 or 6/6 vision. It is common for patients after laser eye surgery to see better than 20/20 or 6/6 vision.
Measuring visual acuity is only one small part of an eye examination and many other aspects have to be assessed before deciding if an eye is healthy and ‘normal’. The only way to decide if your eyesight is normal is to have an appropriate eye examination.Back