What is what prescription is legally blind?

There's no single prescription that legally defines blindness. Legal blindness is determined not by a specific prescription (like -10.00 diopters), but rather by visual acuity and field of vision.

Specifically, in the United States, legal blindness is generally defined as:

• Visual acuity: 20/200 or worse in the better-seeing eye with correction (glasses or contact lenses). This means that a person with 20/200 vision can only see at 20 feet what a person with normal vision (20/20) can see at 200 feet. Even with the best correction, they still have significantly impaired vision.

• OR A field of vision of 20 degrees or less. This refers to the extent of the area a person can see while looking straight ahead. A normal field of vision is much wider.

It's important to note:

• Variations exist: The specific definition of legal blindness may vary slightly from state to state, or between different countries.
• Prescription strength is not the only factor: While a very high myopic (nearsighted) or hyperopic (farsighted) prescription might contribute to legal blindness, it's the resulting visual acuity and field of vision that ultimately determine the legal classification. Other eye conditions like macular degeneration can cause legal blindness even with a relatively mild prescription.
• Legal blindness is a legal definition: It's primarily used for access to disability benefits and services, not a purely medical diagnosis.

Therefore, focusing solely on a prescription number is insufficient to determine legal blindness. A comprehensive eye exam by an ophthalmologist or optometrist is necessary to assess visual acuity, field of vision, and overall eye health to determine if someone meets the criteria for legal blindness.