Visual acuity is a measure of the spatial resolving power of the visual system; it indicates the angular size of the smallest detail that can be resolved. Visual acuity is measured for various purposes. When determining the appropriate eyeglasses, the corrective lens power that permits the best visual acuity is prescribed. In the diagnosis and monitoring of eye diseases that may affect vision, changes of visual acuity are often taken to indicate the presence and magnitude of change in the eye condition. Visual acuity measurements are also used by some licensing authorities and employers as eligibility criteria for some occupations (e.g., airline pilot, police officer) and activities (e.g., driving). Visual acuity has traditionally been used as the primary indicator of the magnitude of functional impairment due to vision loss.

20/20 is commonly used for a pilot’s license and 20/40 for a driver’s license, 20/80 may be used for special educational assistance and 20/200 for tax benefits in the U.S.A.

Setting of visual acuity eligibility levels is a policy decision, not a scientific one. Administrators like to have simple eligibility cut-offs.

Good visual acuity is important for a variety of everyday tasks in the workplace, but probably is most important for reading text and interpreting symbols. The visual acuity demand for a given task depends on the minimum size of the detail in the task and the observation distance. For example, a person with good visual acuity might be expected to recognize faces at about 20 meters. To recognize the same faces, a person with poor visual acuity would have to get significantly closer.

In the workplace, there are several tasks in which it is important to see fine details. Some examples are reading labels, gauges, and dials; inspecting products for cracks, scratches, and foreign material; and visually guided manipulation, as in needle-threading, surgery, and fine assembly tasks. When driving, good visual acuity is important for recognizing environmental landmarks, avoiding small obstacles, and reading highway signs during driving.

A person's "uncorrected" vision refers to the visual acuity when no glasses or contact lenses are used. The "best corrected" vision is the visual acuity with the best glasses or contact lens prescription for that person. Each eye is usually tested separately, although the vision may be slightly better when both eyes are tested together.

The notation of visual acuity is written as a fraction, with normal vision being 20/20 (twenty twenty vision). At a 20 foot distance, (the top number in the fraction, or testing distance), a person with normal vision should be able to read the small 20/20 line on an eye chart. The smallest line that you can read on the chart is your visual acuity.
If larger lines than the 20/20 line are all that can be read, the visual acuity may be 20/30, 20/60, etc. The larger the second number is, the worse is the vision. A person with 20/200 vision would have to come up to 20 feet to see a letter that a person with normal vision could see at 200 feet! Similarly, if the vision is 20/10, it means that the vision is better than normal.

A person with 20/10 vision can read a letter at 20 feet that a person with normal vision would have to come up to 10 feet to read. Eye charts in offices are calibrated for different test distances, so that rooms do not have to be 20 feet long.
Alternative forms of visual acuity notations are shown in this conversion table. (Table from: National Research Council (2002) Visual Impairment. Washington, DC: National Academy Press)

Certain visual acuities have special significance. Some of these are:
•20/20 vision is considered normal vision
•20/40 vision in at least one eye is the vision required to pass the driving test
•20/200 vision or worse is the legal definition of blindness

The chart below may be used to estimate your vision online.

Vision Testing Instructions

The blue "standardization bar" must be measured in centimeters on your computer screen or printout. You should then stand in a position back from your computer screen so that your eye is this same number of feet from the screen. i.e. if the blue bar measures 9 1/2 centimeters in length, you should stand back so that your eye is 9 1/2 feet from the screen.

Vision testing should be performed on each eye separately, wearing distance eyeglasses if required. There are six lines on the screen, you should note the last line number in which you can read most (50-75%) of the letters.

The middle letter of the top line is equivalent to 20/200. A person unable to read this letter with best eyeglass correction is considered legally blind. The "T" and the "B" on the top line are equivalent to 20/100. The second line is 20/50. Best corrected acuity in the range between 20/50 and 20/100 is considered disabling in occupations which require work with numbers or extensive reading. The third line is equivalent to 20/40, it is the "driver's test" line. You must be able to read most of the letters on this line in order to obtain an unrestricted drivers license in most US states.

The bottom three lines represent 20/30, 20/20 and 20/10. These are considered to be "normal vision" with the 20/20 line being the traditional normal vision line.

It is generally felt that the maximum visual acuity of the eye is around 20/15 or 20/16. This means that if you are able to read the bottom line, you are probably a good guesser. This vision test can only be considered a screening test. A variety of factors such as lighting, glare, monitor quality may have an effect on the results of this test. (Vision test reproduced with permission of Ernest Sutcliffe, MD)
REFERENCES:
Colenbrander A. – Aspects of Vision Loss – Visual Functions and Functional Vision. This paper appeared in Volume 5, Issue 3 of “Visual Impairment Reasearch”, the journal of the International Society for Low Vision Research.

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