Visual Acuity Test
The Visual Acuity Test represents the smallest size that can be reliably identified. Visual acuity (VA) is one of many visual perception (eye-based sensory) abilities and is defined as the behavioral ability to resolve fine image details. VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. The Visual Acuity Test represents the smallest size that can be reliably identified. VA is the most common clincal measurement of visual function.
Visual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or Tumbling E.
In some countries, acuity is expressed as a vulgar fraction, and in some as a decimal number.
Using the foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the metre, visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6 vision is equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1.0 is equal to 20/20.
LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss in the visual acuity test ; positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is rarely used clinically; it is more frequently used in statistical calculations because it provides a more scientific equivalent for the traditional clinical statement of “lines lost” or “lines gained,” which is valid only when all steps between lines are equal, which is not usually the case.
If a visual acuity test measures 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from the same distance. If a person has a visual acuity of 20/40, that person is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3). Recent developments in optometry have resulted in corrective lenses conferring upon the wearer a vision of up to 20/10. Some birds, such as hawks are believed to have an acuity of around 20/2, which is significantly better than human eyesight.
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