Visual acuity is an aspect of eye health, often simplified by “20/20 vision.” Many mistakenly believe “20/20” refers to a specific corrective lens prescription. However, this number measures the clarity or sharpness of distant vision. Understanding what 20/20 signifies and how it differs from an eyeglass prescription clarifies common misunderstandings about eyesight.
Understanding 20/20 Vision
The term “20/20 vision” represents a standard benchmark for normal visual acuity, the clarity or sharpness of distant vision. This measurement uses a Snellen eye chart, displaying rows of letters that decrease in size. Individuals stand 20 feet from this chart during an eye examination.
The numbers in “20/20” indicate that a person can clearly see at 20 feet what someone with normal vision can also see at 20 feet. If someone has “20/40 vision,” it means they must be as close as 20 feet to see what a person with normal vision can see clearly from 40 feet away. Conversely, having “20/15 vision” suggests that an individual can see at 20 feet what most people can only see clearly from 15 feet, indicating better-than-average visual acuity. This fractional notation quantifies how well one’s distance vision compares to a standardized norm.
More Than Just 20/20
While 20/20 vision indicates normal sharpness at a distance, it does not encompass all aspects of healthy eyesight. Achieving 20/20 visual acuity does not equate to “perfect vision,” as it only measures a specific part of overall visual ability. Many other visual skills contribute to how well a person sees and interacts with their environment.
These additional visual skills include peripheral awareness, the ability of the eyes to coordinate, depth perception, color vision, and the eyes’ focusing ability for various distances. 20/20 vision does not assess these components. An individual can have 20/20 vision but still experience other visual challenges or underlying eye health issues like glaucoma or cataracts.
From Measurement to Prescription
Visual acuity measurements like 20/20 are distinct from an eyeglass or contact lens prescription. A prescription defines the specific optical power needed to correct vision, measured in units called diopters. These diopter values, which can be positive or negative, specify the strength of the lenses required to help light focus correctly on the retina. For instance, negative diopter values typically correct nearsightedness (myopia), where distant objects appear blurry, while positive values address farsightedness (hyperopia), making close-up objects clearer.
An eye care professional determines this prescription through a comprehensive examination. Corrective lenses work by altering how light enters the eye, ensuring it converges accurately on the retina to improve visual acuity. It is possible to have 20/20 distance vision but still require glasses for other conditions, such as presbyopia. Presbyopia is an age-related condition that causes difficulty focusing on near objects, necessitating reading glasses even if distant vision remains clear.