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.

Unlike prey birds which have 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 distinct. If you can see clearly the fourth line of letters from the bottom, your vision acuity is 20/20, with 20/15, 20/10 and 20/5 below that. 20/20 vision has another explanation: the vision most people can achieve at 20 feet. During a vision test, the eye chart is always placed at 20 feet away from you, or is situated behind your chair with the help of a mirror to simulate the 20 feet distance. If you can only read the top one letters, your vision is poorly 20/200. The lowest best-corrected visual acuity to get a driver’s license in US is 20/40.

For children who can not recognize alphabet, illiterate people, and those are too shy to read letters aloud, a “tumbling E ” eye chart can be used, which consists of capital E characters, which are in four different directions but in same sizes with 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 direction of the letters with their fingers: left, right, up or down.

Eye chart testing can only measure visual acuity and determine whether you need prescription glasses. Peripheral vision, depth perception, color perception, contrast sensitivity and other items are beyond it.

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