In order to be considered legally blind, a person must have a score of no better than 20/200 on a vision test while wearing corrective lenses, according to WebMD. The test may also be administered with a restricted field of vision, but the results must be the same.
Those with test results ranging from 20/70 to 20/200 while wearing corrective lenses are considered partially sighted, says WebMD. If a person scores 20/200 without lenses but gets a better score while wearing corrective lenses, he is considered partially sighted as well and not legally blind. Partially-sighted people are able to improve their vision through corrective measures, according to Wikipedia.