Eye charts for visual acuity testing
Currently, your visual acuity can be tested using the Snellen eye chart, invented by a Dutch eye doctor in the 1860s. The typical Snellen chart consists of 11 rows of capital letters, which are gradually reduced in size. And there is currently another similar chart using tumbling E.
Evaluate visual levels using Snellen eye chart
Unlike prey birds which have visual acuity of 20/5, only a small human population can achieve 20/10 visual acuity. There is a correspondence between these vision terms and the lines on the chart that you can distinguish. If you can see clearly the letters at fourth line from the bottom, your vision acuity is 20/20, and vision of 20/15, 20/10 and 20/5 are corresponding to letters at lines below. People who have received a simple eye chart test would know clearly that letters arranged at the top line are the biggest. So, if you can just see letters starting from the fifth line, your eyesight is weaker than normal.
What does 20/20 vision mean?
There is another explanation for 20/20 vision acuity: the vision most people can achieve at 20 feet away. More specifically, the Snellen eye chart is always placed at 20 feet away from you. If there is inadequate space in the room, the eye chart will be situated behind your chair with the help of a mirror which will simulate the 20 feet distance. At such a distance, most people can see letters at the fourth line. And this is the average visual acuity. However, if you can only read the top one letters, your vision is poorly 20/200. Another baseline is that the lowest best-corrected visual acuity to get a driver’s license in US is 20/40.
Tumbling-E eye chart is an alternative
In addition to the standard Snellen eye chart, there is a variation. For children who can not recognize alphabet, illiterate people, and those too shy to read letters aloud, a “tumbling E” eye chart can be used. This kind of eye chart consists of capital E characters, which are in four different directions but in same sizes as a standard chart. With this tumble E chart, the test results are the same as those from a standard eye chart. But people can simply show the directions of the letters with their fingers: left, right, up or down.
The limitations of eye chart test
It is necessary to point out that eye chart test can only measure visual acuity and determine whether you need prescription glasses or contact lenses. As clearly noticed, this is a very simple eye test, rather than a comprehensive eye examination. Eye chart test can not assess people’s peripheral vision, depth perception, color perception, contrast sensitivity and more complex items.