If you're legally blind, your vision is 20/200 or less. By this definition, you're legally blind if your better eye — when using a corrective lens — has a central visual acuity of 20/200 or lower, or field of vision of no more. The definition of legal blindness was developed as a guideline to help people receive government assistance, such as Social Security disability benefits. What is legally blind? The formal description of legal blindness is: “Central visual acuity 20/200 or less in the better eye with best correction, or. Legal Blindness is not a medical term but a guideline. “Legal blindness” is a definition used by the United States government to determine.