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Since Europe and most parts of the world use the metric system, the World Health Organization uses meters in its definitions. In the United States, visual acuity vison is measured at a distance of 20 feet. For example, 20/20 means you can see at 20 feet what the normal person sees at 20 feet. 20/200 means you can see at 20 feet what a normal person can see at 200 feet.

In Europe and the rest of the world visual acuity is measured at a distance of 6 meters.

What is Normal Vision?

“Normal vision” is therefore defined as a distance visual acuity of 6/6 (or 20/20 in US terms) with a full visual field of 180 degrees. ‘Distance visual acuity is that most central part of vision that is used to see objects at a distance. “6/6 or 20/20” means they can see a specific line of letters when they are standing or seated 20 feet or 6 meters away from an eye chart.

The loss of visual acuity causes blurred vision and you cannot see objects at a distance clearly.

Snellen Eye Chart

The Snellen Eye Chart is the chart most commonly used by ophthalmologists and optometrists to measure a person’s distance visual acuity. The chart is used to measure how well you can see at a standing or seated distance of 20 feet away from the chart. For example, if the eye doctor tells you your distance visual acuity is 20/20 this is determined to be ‘normal’ vision. However if your distance visual acuity is measured at 20/200 this means you can see at a distance of 20 feet what a person with normal vision can see at 200 feet. If you are told your measurement is 20/400 this means you can only see the big “E” on the eye chart, whereas a person with normal vision would be able to see this at 400 feet.

The terms 'low vision" and 'partial sight' mean the same thing. In the USA and many other countries around the world the term ‘low vision’ is more commonly used, whereas in England, the term ‘partial sight’ is more common.

Low vision services are examinations conducted by an optometrist or ophthalmologist who has received additional training in the area of low vision. Both the American Optometric Association [links open in new window] and the American Association of Ophthalmology give formal recognition to those optometrists and ophthalmologists who have gained additional qualifications and experience in the area of low vision.

Certified Low Vision Therapists (CLVTs) often work in close partnership with the optometrist or ophthalmologist, and provide training in how to use low vision optical or non-optical devices in practical everyday situations.