What is 20/20 Eyesight?
Since the mid-1800s, the term “20/20 vision” has been used to characterize the standard of average, or normal, visual acuity. The 20/20 fraction is based on a measurement system known as the Snellen Chart. This chart, which consists of horizontal lines of black letters that decrease in size from top to bottom on a white background, measuresÂ visual acuity, or the ability to distinguish an object from the background. The Snellen Chart may be recognized as theÂ familiar block-letteredÂ “EFPTOZ” poster hanging in manyÂ ophthalmologyÂ andoptometryÂ offices.
To measure distance visual acuity using the Snellen standard, patients stand 20 feet â€“ the numerator, or top number in the fraction â€“ away from the chart. The patient then reads the smallest line on the chart that they are able to. The denominator â€“ or the bottom number in the fraction â€“ indicates the comparative distance in feet at which a person with normal eyesight could read the line. For instance, someone with 20/80 vision (below-average vision) can see at 20 feet of distance what a normal-sighted person can see at 80 feet of distance. Someone with 20/15 vision (above-average vision) can see at 20 feet what a normal-sighted person can see from 15 feet away.Â
Measured in this way, visual acuity can be broken down into the following broad categories:Â
History of Visual Acuity Measurement Tools
The Snellen Chart was developed by Dutch ophthalmologist Hermann Snellen in 1862. As he created the chart, Snellen drew upon a long history of visual evaluation methods. Though the measuring of visual acuity is believed to date back thousands of years to ancient Egypt, one of the strongest historical references is to the work of the SpaniardÂ Benito Daca de Valdes. In 1623, this early clinical ophthalmologist tested his patients' vision by having them count mustard seeds at different distances in order to determine need for convex or concave corrective lenses.Â
A century later, in the 1700s, the task of choosing the correct lenses was left up to patients themselves. Often, people chose corrective lenses based on their age, as the general assumption was that vision deteriorated at the same rate with age for everyone. In the 1850s, a Dutch physiologist named Franciscus Donders advocated for a standard testing mechanism in the form of a lettered chart. He asked Snellen for help, and the first vision acuity chart was created.Â
While the measurement techniques developed by Snellen and Donders are still relevant today, adaptations of their work recent years include:Â
Effects of Aging on Visual Acuity
In addition to his work on acuity measurement with Snellen, Donders was one of the first scientists to study the effects of aging on the eye. In 1862, he commissioned a population study to determine how acuity decreases over time. Recent studies closely mirror the results of Donders' early work.
Modern research shows that almost all people begin to lose visual acuity between the ages of 40 and 60. Of these, more than 32 million in the U.S. are myopic, or near-sighted, and about 12 million are hyperopic, or far-sighted. Both people with myopic and emmetropic â€“ or normal â€“ vision tend to grow more hyperopic with age, due to decreasing power in the crystalline lens, a clear lens that helps the cornea focus light into the retina.Â
By age 75, 48% of people are hyperopic, while only 37% retain emmetropic vision. Other common age-related changes include a tendency toward shifting astigmatism, or misshapen corneas; a decreased amount of light passing through the retina; a reduction of visual acuity when glare is present; and decreasing sensitivity toward visual contrast, also known as low-contrast visual acuity.
Restoring 20/20 Eyesight
A number of modern devices and medical procedures, includingÂ eyeglasses,Â contact lensesÂ and surgeries such as LASIK, can restore vision to 20/20. Conservative approaches to overall eye health, such as the use of natural nutritional supplements, can also help to maintain existing 20/20 vision by supportingÂ macular health, eye hydration and lens clarity.