Needing a strong glasses prescription does not automatically qualify a person as legally blind. While seeing the world as a blur without corrective lenses can feel like a profound visual impairment, legal blindness is a formal designation. This status is based on precise medical measurements used to determine eligibility for specific government benefits and services. It measures the best vision achievable, not how poor uncorrected vision may be.
The Official Criteria for Legal Blindness
The United States government defines legal blindness through two specific measures of sight in the better-seeing eye. The first criterion relates to the clarity or sharpness of central vision, known as visual acuity. A person meets the definition of legal blindness if their visual acuity is measured at 20/200 or less. This measurement is typically taken using a standardized eye chart, like the Snellen chart, and means that the individual must stand at 20 feet to clearly see an object that a person with normal vision can see from 200 feet away.
The second way to qualify involves the scope of a person’s peripheral vision, known as the visual field. Even if central visual acuity is better than 20/200, they can still be classified as legally blind if their visual field is severely restricted. This restriction is defined as subtending an angle no greater than 20 degrees. This limitation is often described as having “tunnel vision,” resulting in a very narrow field of view.
The Critical Role of Corrected Vision
The definitive answer rests entirely on a measurement called best-corrected visual acuity (BCVA). Legal blindness is specifically determined after a patient has been given the best possible conventional correction, such as prescription glasses or contact lenses. If a person’s vision is worse than 20/200 without glasses but improves to 20/100 or better when wearing them, they are not considered legally blind.
The determination requires the eye doctor to use a refraction process to find the most accurate lens prescription that yields the sharpest vision possible. For instance, a patient might have uncorrected vision of 20/400, but if glasses correct that vision to 20/40, their BCVA is 20/40, and they are not legally blind. The diagnosis is reserved for individuals whose vision cannot be restored to better than 20/200, or whose field of vision cannot be widened beyond 20 degrees, even with the most advanced conventional corrective measures. This focus ensures the designation is based on permanent, uncorrectable vision loss.
Differentiating Severe Visual Impairment
Many people who experience poor vision without glasses are dealing with a refractive error, such as myopia (nearsightedness), hyperopia (farsightedness), or astigmatism. A refractive error occurs when the shape of the eye prevents light from focusing correctly on the retina, resulting in blurred vision. These conditions are common and are typically fully remedied with corrective lenses, meaning they do not lead to the permanent status of legal blindness.
Legal blindness and severe visual impairment, sometimes called “low vision,” are distinct categories. Low vision describes vision loss that cannot be fully corrected by glasses, contact lenses, medicine, or surgery, but is not severe enough to meet the 20/200 legal standard. People with low vision often have visual acuity between 20/70 and 20/200 after correction, requiring specialized aids like magnifiers. Legal blindness, in contrast, often results from permanent eye diseases like advanced glaucoma, macular degeneration, or diabetic retinopathy, which cause irreversible damage to the eye structure.