Visual acuity is a measure of the clarity or sharpness of a person’s vision, typically tested at a distance. This measurement is often expressed as a fraction, comparing a person’s sight to the benchmark for normal vision. That standard is widely accepted as 20/20 vision. Understanding the meaning of these fractions is the first step in assessing one’s own visual capability.
The Snellen Scale Explained
The fraction used to describe visual acuity is based on the Snellen chart. This measurement system uses two numbers to represent vision, separated by a slash. The numerator, the first number in the fraction, indicates the standardized distance in feet a person is standing from the eye chart during the test, which is almost always 20 feet in the United States.
The second number, the denominator, represents the distance in feet at which a person with normal, or 20/20, vision can clearly read the same line of letters. If a person has 20/40 vision, they must move to 20 feet away to see what a person with 20/20 vision can see from 40 feet away.
The Snellen fraction is essentially a comparison, indicating how much closer a person needs to be to perceive the same detail as someone with unimpaired vision. The system measures only the clarity of distance vision and does not assess other aspects of sight, such as peripheral vision, depth perception, or color vision. The use of 20 feet as the testing distance is a practical standard, but the same measurement can be expressed in meters, such as 6/6 vision.
Defining 20/80 Vision in Practical Terms
Having 20/80 vision means a person must be 20 feet away from an object to see it with the same clarity that a person with normal 20/20 vision can see from 80 feet away. This level of sight indicates a reduced visual acuity where distant objects appear significantly blurred. For example, to read a road sign clearly, a person with 20/80 vision would need to be four times closer than a person with 20/20 vision.
In real-world terms, this visual impairment can make tasks requiring clear distance vision challenging, such as recognizing faces from across a room or viewing a television screen. The difficulty is especially pronounced when lighting conditions are poor or when trying to focus on objects at a distance.
This score of 20/80 falls within the range considered a moderate visual impairment or low vision. When comparing 20/80 to other measurements, it is a significant drop from 20/40 vision, which is often the required minimum for driving in many jurisdictions. However, it is still substantially better than 20/200 vision, which is the threshold often used in the United States to define legal blindness.
Implications and Correction
A diagnosis of 20/80 vision typically suggests the presence of a refractive error, which is the most common cause of reduced visual acuity. These errors occur when the eye cannot focus light correctly, and they include conditions like myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. While refractive errors are the most frequent culprit, other underlying issues like cataracts, glaucoma, or macular degeneration can also lead to this visual score.
20/80 vision caused by a refractive error is highly correctable, often improving to 20/20 or better with the use of corrective lenses. An eye care professional will prescribe eyeglasses or contact lenses to refocus light onto the retina, restoring clear distance vision. For some individuals, vision correction procedures like LASIK or PRK may also be viable options for a more permanent solution.
It is important to note that 20/80 vision is not considered legal blindness, which is defined as a visual acuity of 20/200 or worse in the better-seeing eye, even with correction. Despite not being legally blind, 20/80 uncorrected vision may still prevent a person from passing the minimum vision screening required for an unrestricted driver’s license in many areas. Regular eye examinations are necessary to determine the cause of the reduced vision and to ensure any underlying eye conditions are detected and managed early.