In the United States, a patient is legally blind if they have a visual field diameter of what degrees or less in the better-seeing eye?

• A. 10 degrees
• B. 20 degrees
• C. 30 degrees
• D. 40 degrees

Answer: (B)

A person is considered legally blind in the United States when the visual field diameter in the better-seeing eye is 20 degrees or less. Normal vision spans a very wide field, so a 20-degree diameter represents a severe constriction—often called tunnel vision—which is what triggers the legal-blindness designation regardless of how well the central vision is preserved with correction. Among the options, 20 degrees is the threshold defined for this criterion; larger fields (30 or 40 degrees) do not meet the standard, and a smaller field like 10 degrees would also qualify, but the commonly cited threshold used for the definition is 20 degrees.