So what exactly is 20/20 vision?

It’s 2020 and the new year has led to lots of media references to 20/20 vision. The implication is usually that such vision is perfect, but this is too simplistic. So what exactly is 20/20 vision?

The term 20/20 and similar fractions 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).

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.

So it is quite possible to see better than 20/20. In fact, most people with young, healthy eyes are capable of identifying at least some of the letters on the 20/15 (6/5 in the UK) line or even smaller letters on the Snellen chart.

Now you know …


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