Vision of 20/40 is worse than standard vision and sometimes requires glasses, depending on the exact nature of the vision problem, according to Eye Care Fun. Someone with vision of 20/40 can see objects from 20 feet away that someone with normal vision can see from 40 feet away.
Normal vision is considered to be 20/20, Eye Care Fun notes. An indivdiual who can't achieve vision of at least 20/200 with the assistance of glasses is defined as legally blind in the United States. Someone is also considered to be legally blind if their peripheral vision is less than 20 degrees.
Vision, also known as visual perception, is the interpretation of the environment through information within light, according to Wikipedia. Academic fields including neuroscience, biology and psychology conduct research on the parts of the body involved in vision.