What is Vision Testing

Vision

Vision is a broader term than visual acuity or eyesight. In addition to clarity of sight or simply a description of the ability to see, the term "vision" all interactions between the eyes and the brain, and all neurological processes that take place in the brain to make the sense of vision possible.

Also, unlike simple eyesight or Snellen (high contrast) visual acuity, measures of vision include contrast sensitivity, the ability to track moving objects with smooth and accurate eye movements, colour vision, depth perception, focusing speed and accuracy, and more.

Because of the broader nature of the word “vision,” what is commonly called “20/20 vision" should really be called "20/20 visual acuity" or “20/20 eyesight.”

What is 20/20?

The term "20/20" and similar fractions (such as 20/60, 20/40 etc.) are visual acuity measurements. They also are called Snellen fractions, named after Herman Snellen, the Dutch ophthalmologist who developed this eyesight measurement system in 1862.

In the Snellen visual acuity system, the top number of the Snellen fraction is the viewing distance between the patient and the eye chart. In the United States, this distance typically is 20 feet; in the UK it is 6 metres (therefore 20/20 is equal to 6/6).

At this testing distance, the size of the letters on one of the smaller lines near the bottom of the eye chart has been standardised to correspond to "normal" visual acuity — this is the "20/20 (6/6)" line. If you can identify the letters on this line but none smaller, you have normal 20/20 (6/6) visual acuity.

The increasingly larger letter sizes on the lines on the Snellen chart above the 20/20 (6/6) line correspond to worse visual acuity measurements (20/25; 20/32; etc.); the lines with smaller letters below the 6/6 line on the chart correspond to visual acuity measurements that are even better than 20/20 vision (e.g. 20/16; 20/10).

The single big "E" at the top of most Snellen eye charts corresponds to 20/200 visual acuity. If this is the smallest letter size you can discern with your best corrective lenses in front of your eyes, you are considered legally blind.

On most Snellen charts, the smallest letters correspond to 20/10 visual acuity. If you have 20/10 visual acuity, your eyesight is twice as sharp as that of a person with normal (20/20) vision.