– Ed

It is a persistent urban legend that 6/6 would represent normal, average or even perfect vision. This is not so.

Herman Snellen, a Dutch ophthalmologist, published his Snellen Chart in 1862. To this day, the Snellen Chart is still used to quantify how well we can see.

The simple interpretation is that, normal vision is 6/6, meaning you can read a letter of a standard size, at a distance of 6 meters. If you have 6/9 vision, it means that you can read something from a distance of 6 metres, that a normal person is capable of reading from a distance of 9 metres. At the other end of the scale; 6/36 means you can read a letter at 6 meters, that a normal person can read at 36 meters.

It is a persistent urban legend that 6/6 would represent normal, average or even perfect vision. This is not so. Snellen deliberately chose his reference standard (5 min of arc) as a size that is “easily recognised by normal eyes.” Thus, almost all normal eyes will equal or exceed the reference standard. If 6/6 equalled average acuity, half of the population would fall short of 6/6, since that is the definition of average.

Visual acuity values are understood best by the following simple rule. On a Snellen Chart we determine the line that the person can just recognize. If that line is twice as large as the reference standard (6/6), we state that, that person’s Magnification Requirement (MAR) is 2x. If the Magnification Requirement is 2x, the visual acuity is 1/2 (6/12). Similarly, if the Magnification Requirement (MAR) is 5x, the visual acuity is 1/5 (6/30); if MAR = 10, visual acuity = 1/10 (6/60), and so on. The MAR and the visual acuity scale are opposites. A high MAR value indicates low or poor visual acuity; a low MAR value indicates good acuity. Their relationship is also true in reverse. A patient with 20/60 (1/3) visual acuity needs 3x magnification to reach the reference standard. This can be achieved with large print that is 3x normal, with a 3x magnifier or with a 3x telescope. Since normal vision is better than 6/6, some extra magnification is desirable for comfortable and sustainable performance.

Eye charts can be configured in various ways, but generally, if during an eye test you can read the big E at the top, but none of the letters lower than that, your vision is considered 6/60. That means you can read a letter at 6 meters that people with “normal” vision can read at 60 meters. If your best-corrected visual acuity (meaning, your best distance vision with glasses or contact lenses) is 6/60 or worse, you can be considered legally blind. To get a driver’s license, your best corrected visual acuity must be 6/12 or better.

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