20 / 20 Vision
Eye doctors frequently use the term 20/20 to describe our vision, but what exactly do they mean and how is it measured? The numbers do not actually relate to each eye respectively, as you might logically expect. 20/20 refers to how well a person can see at a distance of 20 feet. It is a term used to define normal visual acuity; the clarity and sharpness of an object at a distance of 20 feet. Often eye tests are performed in small consulting rooms that are certainly not 20 feet in length. Fortunately, with modern technology, the doctor can use a series of mirrors to project the letters onto the screen to give the same effect as a distance of 20 feet. If the patient sees less than 20/20 vision, the doctor will work out the cause. Once the problem has been diagnosed, glasses or contact lenses may be prescribed so that the desired 20/20 vision can be attained.
What the Numbers Mean
In a standard eye test the letters get progressively smaller to assess the patient's vision. The smaller the letter, the smaller the second number will be. The smaller the second number, the better vision you have, so 20/20 is not actually perfect vision; it is just what a normal person can see at that standard distance. For example, if you have 20/100 vision it means that at 20 feet you can only see what a normal person can see at 100 feet, so your eyesight would be very poor. 20/10 vision would be an excellent result as you would have twice the normal acuity, being able to read at 20 feet what a normal person would be expected to read at 10 feet. This is usually the maximum acuity the human eye can reach without using aids such as binoculars. Interestingly a hawk, which is known for its sharp vision is believed to have an acuity of 20/2 which is far better than unaided human eyesight. On the other side of the scale a newborn baby has an estimated visual acuity of 20/400, typically (not in all cases) developing to 20/20 by the age of 2 years old. Obviously other tests have to be used to assess those who are too young or handicapped to read a letter chart.
Definition of Blindness
If a person cannot achieve 20/200 vision or better in their best eye, even with glasses, they are considered legally blind in the USA. Also someone whose visual field is narrower than 20 degrees meets the definition of being legally blind.
The test and the term are measured in feet, but in Europe and many other countries distance is only measured in meters. The metric equivalent is 6/6 vision as 6 meters is the approximate equivalent to 20 feet.
The Snellen Chart
The standard eye chart used for visual acuity testing is known as a Snellen chart and was developed in 1862 by Hermann Snellen, although it has been modified several times since. The big E at the top measures 20/200 vision so reading that is no big deal. The chart has rows of letters which get progressively smaller. 20/20 vision is actually line 8 so you need to be able to read quite a few of the upper lines to have even reasonable eyesight. Eye exams test one eye at a time as our eyesight is different in each eye.
Other Factors in Determining Eyesight
As mentioned previously, 20/20 vision is a simple measurement and attaining that measurement does not mean you have perfect eyesight. There are many other factors to take into account. Vision skills also include side vision, or peripheral awareness. Someone with tunnel vision may be able to read those eye charts at 20 feet but with no side vision there is clearly something wrong. Other tests may assess the factors which can be used to diagnose more serious problems including eye disease. Eye coordination, the perception of depth and focusing ability are all part of good vision. Being able to discern color also can affect our eyesight so 20/20 is good, but it is only part of measuring good eyesight.
Farsightedness and Nearsightedness
Some people may be able to see perfectly at a distance of 20 feet; however they cannot focus on the small print on a label and may have trouble reading a newspaper. They are deemed to be farsighted. They can see distant objects clearly, but not close up. A 20/20 eye test does not allow for this. Conversely, some people cannot see objects clearly at 20 feet, but they do not realize they have a sight problem. They can read even the smallest print close up, and think they can see sufficiently well to drive. They are classified as nearsighted. An eye test may show that they do need glasses for distance. It takes a qualified optometrist and the use of several examinations and tests to perform a full eye test. Now we know that being told we have 20/20 vision is good news, but it is only part of the overall picture.