Understanding Common Visual Acuity Tests
Confusing medical terminology can often keep us from understanding what's going on in our bodies, especially at the doctors office, but a little information can go a long way. If you understand the basic visual acuity tests you'll be given by your optometrist, you'll be better able to interpret the results, recognize problems, weigh your treatment options and know when to ask questions.
Visual acuity tests measure how well we can see at different distances. It tells an eye doctor what is required to best improve any defects using eyeglasses, contact lenses or other means. How can we tell if our eyesight is as good as everyone else’s? A number of simple eye tests were created to compare one person’s sight with what can reasonably be expected, taking into account many different factors.
The Snellen chart
Most people are familiar with the Snellen chart, though they probably don't know it by name. (Hint: it's the one with the giant E at the top.) Developed in 1862 by Dutch eye doctor Hermann Snellen, the chart is still seen on the wall of most optometrists' offices today, as well as other places where vision tests are performed, like the offices of the Department Motor Vehicles.
The Snellen chart consists of 11 rows of capital letters, with each row being smaller print than the last. During testing, the patient is asked to stand a specific distance from the chart and read the letters to the doctor. The smallest line that they can see clearly represents the acuity of their vision, with the big E at the top being 20/200 (which as you can imagine, is not very good) and the 8th line down 20/20 vision, which is considered normal. The remaining 3 lines represent better-than-normal vision, with the smallest line being 20/10. If you can read this last line, you have fantastic vision!
Each eye is separately tested as our eyes are not identical and one is often weaker than the other.
Tumbling E chart
For those too young to read or who are illiterate or mentally handicapped, the tumbling E chart is used. It is similar to the Snellen chart, but instead of having different letters, each row is made up of the letter E facing different directions. So rather than read the letters aloud, the patient has to indicate which direction the ‘fingers’ of the Es are facing.
Accuracy with the Tumbling E chart is much the same as with the standard Snellen chart.
What is 20/20 vision
20/20 is a term used to define normal visual acuity; the clarity and sharpness of an object at a distance of 20 feet. 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, on the other hand,would mean you have exceptionally good vision, being able to read at 20 feet what a normal person could only read at 10 feet.
20/10 is usually the maximum acuity the human eye can reach without using aids like binoculars. In comparison, a hawk, which is known for its sharp vision, is believed to have an acuity of 20/2 which is far better than our unaided eyesight. On the other side of the scale, a newborn baby has an estimated visual acuity of 20/400, developing to 20/20 by the age of 2 years old.
To get a driver’s license in the USA you need 20/40 vision or better, with or without glasses. If your visual acuity with the best eyeglasses or contact lenses is still 20/200 or worse than you are deemed legally blind.
Often eye tests are performed in small consulting rooms that are smaller than 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.
Just one part of a complete eye exam
Keep in mind that things like the Snellen or Tumbling E eye charts only test visual acuity, which is just one component of good vision. A full eye examination will cover far more, including peripheral vision, depth perception, color vision and contrast sensitivity. There are also tests which will check your intraocular pressure, the dryness of your eyes and detect other signs of eye disease or cataracts. Even if you have 20/20 vision, you must see your eye doctor regularly (typically once every 2 years) to check for other problems.
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