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What is 20/20 vision? How do we measure vision?

February 3, 2012

When you go to an optical shop or a clinic to get your vision tested, you must have heard the nurse, optometrist or the ophthalmologist telling you that you have 20/20 vision or 6/6 vision. What does that mean?

Most vision charts measure Snellen acuity.  A common Snellen chart has the big “E” or a big number at the top of the chart.  A Snellen chart is probably the most common vision chart found in many optical shops and clinics in Singapore and in many other countries.

Snellen acuity measures your vision as a “fraction.”  20/20 vision is the standard or “perfect” vision. 20 feet equals 6 meters. This means that 20/20 vision is the same as 6/6 vision, just depends on whether you are used to using feet or meters. In Singapore, 6/6 is the visual notation used among doctors and optometrists. The notation 20/20 is used in North America.

20/20 means that you are able to see at 20 feet, what “normal” person sees at 20 feet.  Likewise 6/6 means that you are able to see at 6 meters what a “normal” person sees at 6 meters. Another example, 6/24 (20/80) means that you must stand 6 meters (20 feet) from a target/chart to see the same as the “normal” person standing 24 meters (80 feet) away.

These days, many clinics, including my clinic at Shinagawa LASIK & Eye Centre, use a projector that projects numbers or letters onto the wall. Here is an example of one. You can see the numbers on the right column specifying the visual acuity of the particular row of numbers: 100 feet equals 30 meters, 80 feet equals 24 meters.

Vision Chart projected onto wall

We use the following conventions when a person cannot see the largest letter on the vision chart:

  • Counting Fingers – measuring your ability to count the examiners fingers at a given distance, example CF at 1 meter.
  • Hand Motions – vision is limited to seeing movement, such as a waving hand, example, HM at 1 meter
  • Light Perception – vision is poor, can only see light
  • No Light Perception – no vision, completely black; total blindness

Snellen acuity has many limitations, but it has been the basis for testing visual acuity, for example, it cannot measure distortion, color, glare or blind spot.

Our vision can be recorded as unaided vision (without the aid of glasses or contact lenses) or aided vision (with glasses or contact lenses). A person is considered legally blind when he cannot see 6/60 or better with visual aid. Vision with the most appropriate pair of glasses on is also known as best-corrected vision. A cataract is an example of an eye disease that reduces best-corrected vision. In some people with high myopia or high astigmatism, their best-corrected vision may be slightly poorer than 6/6 even though they have no other eye disease. This means that they cannot see the 6/6 line on the vision chart no matter what power of lenses you place in front of their eyes.

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