In order to be considered legally blind, someone must have vision of 20/200 (6/60) in his or her best eye with correction, and/or have a visual field which is limited to 20 degrees or less, in contrast with the 180 degree visual field enjoyed by people with healthy eyes. This definition means that it is possible for someone to be able to see and to still be considered blind in the eyes of the law.

Governments have come up with a definition for legal blindness for the purpose of being able to determine who should receive special government benefits and assistance for blindness. Someone who is legally blind may be entitled to disability payments and other assistance, such as note takers in college classes, or aids such as seeing eye dogs, canes, and so forth which will help them navigate the world.

Visual acuity is expressed in a format which compares someone's vision to “normal” vision when viewing an object at a set distance in feet or meters. For nations which use feet as a unit of measurement, someone with normal vision is said to have 20/20 vision. In the metric world, the measurement of 6/6 is used. If the second number is smaller, it means that someone has visual acuity which is better than normal, because he or she can stand further away from an object and still see it clearly, while if the second number is larger, it means that visual acuity is worse, because the person needs to stand closer to an object to see it.

In the case of someone with 20/200 (6/60) vision, he or she needs to stand within 20 feet (6 meters) to see an object which someone with normal visual acuity can see from 200 feet (60 meters) away. This has obvious consequences, as it means that a legally blind person has trouble seeing things which other people take for granted, like road signs, traffic lights, and so forth.

People who are legally blind are also more prone to falls and other accidents because they cannot clearly discern their surrounding environment. A person with 20/20 (6/6) vision, for example, would notice a crack in the sidewalk and avoid it, but a legally blind individual might not be able to clearly see the crack, or might not understand what the visual disturbance meant, so he or she could trip and fall.

Many cities have measures in place which are designed to help legally blind individuals, such as warning sounds on crosswalks to alert people to when it is safe to walk, and textured inserts in sidewalks which will let people know that the sidewalk elevation is about to change. Individuals who are legally blind can take advantage of the same tools used by people with no visual perception at all, such as screen readers for using computers, and Braille texts for reading print publications.