![]() The terms “vision” “eyesight” and “visual acuity’ are often used interchangeably. Vision is a more general term including a wider range of visual skills such as contrast sensitivity, the ability to track moving objects with smooth and accurate eye movements, colour vision, depth perception, the ability to focus with speed and accuracy, contrast sensitivity, peripheral awareness or side vision and eye coordination. In normal life objects are both coloured and similar in brightness to their background. Visual acuity is also tested under high contrast conditions the letters on the eye chart are black and the background of the chart is white. In normal conditions our eyes are focusing and responding to both moving and non moving objects at a distance, middle range and up close. During the test you are sitting still and the letters are also stationary, making it a static test. Visual acuity is tested under clinical conditions. The terminology refers to the sharpness or clarity of vision at a distance. Some people can see better than 20/20 and they normally have a higher sensitivity in their retina. Having “20/20 vision” does not necessarily mean that you have perfect vision. When most people say “20/20 vision” they mean “20/20 visual acuity”. The ability of the brain to interpret information received from the eyes. The sensitivity of the nerves in the retina and vision centres in the brain, and How accurately the cornea and lens of the eye focus light onto the retina, Visual acuity is determined by three physical and neurological factors: The single large “E” at the top of the Snellen chart corresponds to 20/200 visual acuity and if this is the smallest letter that you can see with your corrective lenses in front of your eyes, you are classified as legally blind. if you have 20/40 (or 6/12) vision then you will just be able to see something from a distance of 20 feet (6 meters) that a person with perfect eye sight can see from 40 feet (12 meters). The increasingly larger letters above the 20/20 line on the Snellen chart correspond to poorer visual acuity measurements eg. Most young people with healthy eyes can read below this line. The Snellen chart has a 20/20 line and those letters that are smaller below the line correspond to a visual acuity that is better than 20/20 vision, such as 20/15 or 20/10. In New Zealand we use the metric system and refer to it as 6 meters. In North America and America this distance is 20 feet. In the Snellen visual acuity system, the top number of the fraction is the viewing distance between the patient and the eye chart. The visual acuity measurement is assessed using an eye chart measuring system developed by a Dutch ophthalmologist, Herman Snellen, in 1862. 20/20 vision is the term used to express normal visual acuity or the sharpness of your vision. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance.
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