A high eyeglass prescription, such as minus six, often raises concerns about the severity of one’s vision and whether it constitutes legal blindness. The term “legal blindness” is not a medical diagnosis but a strictly regulated designation used for governmental and administrative purposes. This classification is separate from a high prescription, which signifies significant uncorrected vision loss. Understanding the difference between a refractive error measurement and the standard for legal blindness is essential.
Establishing the Criteria for Legal Blindness
The determination of legal blindness in the United States is based on two primary criteria related to a person’s best corrected vision. The first focuses on visual acuity, which must be 20/200 or less in the better-seeing eye, even with the best conventional correction, such as glasses or contact lenses. This 20/200 measurement means that a person must stand 20 feet away to see what a person with normal vision can see clearly from 200 feet away. If corrective lenses improve sight past this threshold, the person is not legally blind.
The second criterion involves a limitation in the field of vision, or peripheral sight. A person qualifies if their visual field is restricted to an angle no greater than 20 degrees, often described as tunnel vision. This statutory definition is used by bodies like the Social Security Administration to determine eligibility for benefits. A person only needs to meet one of these two criteria to be classified as legally blind.
Deciphering Refractive Error Measurements
An eyeglass prescription uses the diopter (D) to measure the optical power needed to correct vision. This measurement indicates how strongly a lens must converge or diverge light to focus precisely on the retina. A negative sign (-) indicates myopia, or nearsightedness.
Myopia occurs when the eyeball is too long or the cornea is too curved, causing distant images to focus in front of the retina. A prescription of -6.00 D is classified as high myopia, representing a substantial refractive error. The magnitude of the diopter number reflects the degree of the error; the further the number is from zero, the stronger the lens needed. This figure measures the eye’s shape and focusing ability, not the functional vision achieved with correction.
The Critical Difference: Prescription Versus Corrected Vision
A -6.00 D prescription, while considered high myopia, does not automatically result in legal blindness. Legal blindness is based on the best corrected visual acuity—the sharpest vision achieved using corrective lenses. Most individuals with a -6.00 D prescription can be corrected to achieve visual acuity significantly better than the 20/200 legal threshold.
If a person with high myopia achieves 20/40, 20/80, or even 20/100 while wearing corrective lenses, they are not legally blind, regardless of the prescription strength. The high prescription simply reflects the severe blurriness of their vision when uncorrected. However, high myopia can indirectly lead to legal blindness.
High myopia increases the risk for several eye conditions that cause irreversible damage, such as myopic macular degeneration, retinal detachment, and glaucoma. If these complications damage the retina or optic nerve, the resulting vision loss may prevent the eye from being corrected past 20/200. In this scenario, the underlying damage caused by high myopia, not the prescription power, leads to the legal blindness designation.