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Visual acuity, also known as clarity of vision or sharpness of vision, refers to a person's ability to see small details. A visual acuity score is the result of a vision test performed by an optometrist or ophthalmologist (opticians, technicians, and nurses can also perform a visual acuity measurement). It is usually expressed as a fraction that measures a patient's ability to see an object from 20 feet away compared to a person with normal vision who can see the object clearly from a longer distance.
For example, 20/20 vision means that you can see an object clearly from 20 feet away. 20/40 vision means that you need to be 20 feet away to see clearly what a person with normal vision can see from 40 feet away.
Visual acuity is dependent on a combination of optical and neural elements, including:
Refractive errors or neural factors usually cause poor visual acuity. Refractive errors are visual impairments that affect the way light is bent, or refracted, in your eyeball, resulting in decreased visual acuity. Common refractive errors include:
Neural factors that can decrease acuity occur in the retina, brain, or the pathway between the two. Examples include:
Most of the time, lower visual acuity can be corrected by eyeglasses, contact lenses, or refractive surgeries.
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Visual acuity is measured by charts, optical instruments, or computerized tests. Eye charts are the most common method of visual acuity testing. They show several rows of optotypes (letters, numbers, or symbols) that get progressively smaller towards the bottom. The most frequently used eye charts used are the Snellen Chart (left) and Tumbling E Chart (right).
An optometrist or ophthalmologist typically performs a visual acuity test. During the eye test, you'll be seated a certain distance from the visual acuity chart. Elements such as distance from the test chart and lighting conditions must be standardized.
The patient will cover their right or left eye and read the chart starting at the top and proceeding downwards until they can no longer distinguish the letters. They will then switch eyes and repeat the process from the same viewing distance. The doctor will use the line with the smallest visible letter size to give you your results.
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Visual acuity is typically measured in fractions or decimals. The first number in the fraction refers to the testing distance, and the second number refers to the distance someone with "normal" vision could see the same details from. Most vision testing in the United States uses the Snellen letter chart, which requires a test distance of 20 feet.
For example, 20/20 vision means that a person can see an object as well as anyone with "normal" vision from 20 feet. If you have 20/30 vision (a lower visual acuity score), it means the details you see from 20 feet away can be seen from 30 feet away by someone with "normal" visual abilities. Basically, the higher the second fraction number, the lower the visual function.
In most other countries, visual acuity is expressed using the metric system. Since 20 feet is equivalent to 6.096 meters, 20/20 vision is equal to 6/6 vision.
The National Vision Research Institute of Australia developed the LogMAR chart (Logarithm of the Minimum Angle of Resolution) in 1976. The Bailey-Lovie chart and ETDRS chart (Early Treatment Diabetic Retinopathy Study) are two charts that use the LogMAR scale. Its design provides a more accurate visual acuity score than other charts. With a LogMAR chart, your visual acuity score is measured in reference to the minimum angle of resolution.
20/20 is considered normal visual acuity. If you have 20/20 vision, it means that you can see objects clearly from 20 feet away. This is expressed as 6/6 in metric, 1.00 in decimal, and 0.0 in LogMAR measurement.
An optometrist or ophthalmologist typically measures visual acuity. Although opticians, technicians, and nurses can also measure it. Your score will be in feet, meters, decimals, or the LogMAR scale.
6/12 is a metric measurement on the Snellen scale. It is equivalent to 20/40 in feet, .50 in decimal, and .30 on the LogMAR scale.
A visual acuity score is a measurement of a person’s ability to see small details with precision. It is also known as clarity of vision or sharpness of vision.
Kniestedt, Christoph, and Robert L Stamper. “Visual acuity and its measurement.” Ophthalmology clinics of North America vol. 16,2 (2003): 155-70, v. doi:10.1016/s0896-1549(03)00013-0
Bailey IL, Lovie JE. New design principles for visual acuity letter charts. American Journal of Optometry and Physiological Optics. 1976 Nov;53(11):740-745. DOI: 10.1097/00006324-197611000-00006.
Raasch TW, Bailey IL, Bullimore MA. Repeatability of visual acuity measurement. Optometry and Vision Science : Official Publication of the American Academy of Optometry. 1998 May;75(5):342-348. DOI: 10.1097/00006324-199805000-00024.
Rabbetts, R. B. (1989). Validity and reliability of visual acuity measurements. Ophthalmic and Physiological Optics, 9(4), 458-458. doi:10.1111/j.1475-1313.1989.tb00957.