The question of whether a high eyeglass prescription, such as a minus seven (-7) diopter, qualifies someone as legally blind is often misunderstood. Many assume a strong corrective lens power automatically equates to a severe visual disability. However, the strength of the corrective lens, measured in diopters, measures the eye’s refractive error, not functional vision loss. Legal blindness is determined by strict criteria regarding how well a person can see with their corrective lenses.
What a -7 Diopter Prescription Means
A negative number on a prescription signifies myopia, commonly known as nearsightedness. This refractive error means light focuses in front of the retina instead of directly on it, causing distant objects to appear blurry. A prescription of -7 diopters is considered high myopia, indicating a substantial degree of nearsightedness.
High myopia usually results from the eyeball being elongated or the cornea having too much curvature. This excessive length requires a strong concave lens, represented by the negative number, to push the focal point back onto the retina. Without correction, a person with a -7 prescription would have significant difficulty seeing clearly beyond a very short distance.
The Strict Definition of Legal Blindness
Legal blindness is a classification used by government agencies to determine eligibility for specific benefits and services. This definition relies on two primary factors: corrected central visual acuity and visual field. Visual acuity measures the sharpness of vision, typically using the Snellen eye chart.
A person is classified as legally blind if their central visual acuity is 20/200 or less in the better eye, even with the best possible correction. The 20/200 term means the individual must be 20 feet away to see what a person with normal vision (20/20) sees from 200 feet. Alternatively, legal blindness is defined if the visual field is restricted to 20 degrees or less, often described as tunnel vision, in the better eye. The standard is based entirely on the remaining functional vision after corrective measures have been applied.
Why Refractive Error is Not the Determining Factor
The prescription strength of -7 diopters measures the eye’s shape and its ability to focus light, not the final visual outcome with correction. If a person with a -7 prescription achieves a corrected visual acuity better than 20/200 (e.g., 20/40 or 20/100) while wearing lenses, they do not meet the threshold for legal blindness. Therefore, the high prescription number is irrelevant to the legal status if the lenses successfully correct the refractive error.
While a -7 diopter prescription is not automatically legal blindness, high myopia increases the risk for sight-threatening eye conditions. The physical elongation of the eyeball stretches and thins internal structures, increasing the likelihood of complications such as retinal detachment, myopic macular degeneration, or glaucoma. If one of these complications causes irreversible damage that prevents vision from being corrected to better than 20/200, the individual would then meet the criteria for legal blindness. The cause of the legal blindness is the disease process, not the -7 lens power itself.