Understanding vision involves terms like “20/20 vision” or “20/200 vision.” These numbers describe visual acuity, which measures the sharpness of your sight. This article clarifies what 20/200 vision signifies, how it is determined, its broader implications, and how it translates into a vision prescription for corrective lenses.
Understanding 20/200 Vision Acuity
Visual acuity is commonly expressed as a fraction, such as 20/20 or 20/200, based on the Snellen eye chart. The first number, typically 20, represents the viewing distance in feet. The second number indicates the distance at which a person with normal vision could clearly read the same line. Normal visual acuity is 20/20, meaning a person can see at 20 feet what someone with typical vision can see at 20 feet.
When someone has 20/200 vision, they must be 20 feet away to see what a person with normal vision can discern from 200 feet away. This signifies a significant reduction in visual sharpness compared to 20/20 vision. For example, a person with 20/200 vision needs to stand much closer to an object to see it with the same clarity as someone with 20/20 vision. This measurement provides a standardized way to describe a person’s uncorrected or best-corrected sight.
How Vision Acuity is Assessed
Visual acuity is assessed during a routine eye examination using a standardized eye chart, most commonly the Snellen chart. This chart features rows of letters that decrease in size from top to bottom. The individual stands or sits a specific distance, typically 20 feet, from the chart.
During the test, each eye is usually tested separately. The patient reads the smallest line of letters they can clearly identify. The last line successfully read determines the visual acuity fraction for that eye. For example, if an individual can only read the large “E” at the top of the chart from 20 feet, their visual acuity might be recorded as 20/200.
20/200 Vision and Legal Definitions
The term 20/200 vision holds specific significance in legal definitions, particularly concerning legal blindness. In the United States, legal blindness is often defined as having a best-corrected visual acuity of 20/200 or worse in the better eye. This means if an individual’s vision in their stronger eye cannot be improved beyond 20/200, even with corrective lenses, they meet the criterion for legal blindness.
The definition also includes individuals with a visual field limited to 20 degrees or less. This threshold determines eligibility for various government programs, services, and benefits designed to assist individuals with severe vision impairment. Legal blindness does not necessarily mean total blindness, but rather a significant visual impairment that impacts daily functioning.
From Acuity to Prescription: Correcting Vision
While 20/200 is a measurement of visual acuity, it is not a prescription itself. An optometrist uses this acuity measurement, along with other diagnostic tests, to formulate a precise vision prescription. This process involves a refraction test, where different lenses are placed in front of the eye to determine which combination provides the clearest vision. This test helps identify specific refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), or astigmatism.
A vision prescription typically includes several numerical values. “Sphere” indicates the overall lens power needed to correct nearsightedness or farsightedness, while “cylinder” and “axis” address astigmatism. These values dictate the curvature and orientation of the lens needed to focus light onto the retina. For individuals with 20/200 vision, their prescription numbers reflect the significant optical power required to bring their vision closer to 20/20.
Corrective lenses, whether glasses or contact lenses, work by altering the path of light entering the eye so that it converges precisely on the retina. For example, in cases of myopia, where light focuses in front of the retina, concave lenses diverge light rays before they enter the eye, allowing them to focus correctly. The precise measurements in a prescription ensure optimal visual correction for the individual’s specific refractive error.