Generality/Definition
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What is visual acuity? Acuity is the measure of the eye's ability to distinguish the smallest identifiable letter or symbol, its details and shape, usually at a distance of 20 feet. This measurement is usually given in a fraction. The top number refers to the testing distance measured in feet and the bottom number is the distance from which a normal eye should see the letter or shape. So, perfect vision is 20/20. If your vision is 20/60, that means what you can see at a distance of 20 feet, someone with perfect vision can see at a distance of 60 feet.
Source: Frequently Asked Questions about Eye Conditions & Eye Care - Kellogg Eye Center (kellogg.umich.edu)
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Causes
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Nearsightedness results in blurred vision when the visual image is focused in front of the retina, rather than directly on it. It occurs when the physical length of the eye is greater than the optical length. For this reason, nearsightedness often develops in the rapidly growing school-aged child or teenager, and progresses during the growth years, requiring frequent changes in glasses or contact lenses. A nearsighted person sees near objects clearly, while objects in the distance are blurred. Farsightedness is the result of the visual image being focused behind the retina rather than directly on it. It may be caused by the eyeball being too small or the focusing power being too weak. Farsightedness is often present from birth, but children can often tolerate moderate amounts without difficulty and most outgrow the condition. A farsighted person sees faraway objects clearly, while objects that are near are blurred.
Source: MedlinePlus Medical Encyclopedia: Normal, near, and farsightedness (nlm.nih.gov)
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Four common refractive errors are:
* Myopia, or nearsightedness - clear vision close up but blurry in the distance
* Hyperopia, or farsightedness - clear vision in the distance but blurry close up
* Presbyopia - inability to focus close up as a result of aging
* Astigmatism - focus problems caused by the cornea
Source: MedlinePlus: Refractive Errors (nlm.nih.gov)
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Diagnosis
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A standard eye chart is necessary to make comparisons and to record people's visual acuity. The most common chart used in most doctors' offices is the Snellen eye chart. In 1862, a Dutch Ophthalmologist, Dr. Hermann Snellen, devised this eye chart. He determined that there was a relationship between the sizes of certain letters viewed at certain distances. A copy of the Snellen chart may be found here. The Snellen eye chart has a series of letters or letters and numbers, with the largest at the top. As the person being tested reads down the chart, the letters gradually become smaller. Many other versions of this chart are used for people who cannot read the alphabet. The Tumbling E chart has the capital letter "E" facing in different directions and the person being tested must determine which direction the "E" is pointing, up, down, left, or right. A Broken Wheel vision test is one that can be used for children or those who cannot read the alphabet and the person being tested must tell which card has the broken wheels on the pictured car. Another type of eye chart that can be used is a picture chart with common pictures of different sizes. In research, another chart, the ETDRS chart, has become the standard. Originally, the chart was used in the Early Treatment of Diabetic Retinopathy Study.
Source: How Visual Acuity Is Measured (mdsupport.org)
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Visual acuity tests may be done:
* As part of a routine eye exam to screen for vision problems. How often you should have routine eye exams changes as you age:
o Once between the ages of 3 and 5
o Before a child begins first grade, then every 2 years
o Once between the ages of 20 and 29
o Twice between the ages of 30 and 39
o Every 2 to 4 years between the ages of 40 and 65
o Every 1 to 2 years after age 65
* To monitor an eye problem, such as diabetic retinopathy.
* To determine if you need glasses or contact lenses to improve your vision.
* After an injury to the eye.
* When you obtain or renew your driver's license or for some types of employment.
Source: Vision Tests (webmd.com)
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Treatment
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Myopia is best treated with eyeglasses and contact lenses which compensate for the elongated shape of the eye allowing the light to focus properly on the retina. As children (and their eyes) grow through the teen years, the condition typically worsens and then levels off in adulthood. During this growing period, new eyeglasses may be needed as often as every six months to correct the problem. There is no scientific evidence that contact lenses or eye exercises stop the progression of myopia. *Refractive surgery* is available as a treatment for myopia but most ophthalmologists-medically trained eye doctors-feel that eyes with simple myopia would best be treated with glasses or contact lenses.
Source: The Canadian Ophthalmological Society - Programs & Services - Eye Conditions, Disorders and Treatments - Myopia (eyesite.ca)
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Illustrations
Information for specialists
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The estimates that I use for approximately what the correction needed for the line of vision seen is: | Snellen | Estimated prescription | | 20/10 | Plano (zero) | | 20/15 | Plano | | 20/20 | Plano to -0.25 | | 20/30 | -0.50 | | 20/40 | -0.75 | | 20/50 | -1.00 to -1.25 | | 20/100 | -1.75 to -2.00 | | 20/200 | -2.00 to -2.50 |
Source: How Visual Acuity Is Measured (mdsupport.org)
Scientific articles:
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