Both eye injury and disease can affect vision. The clarity of vision is called visual acuity, which ranges from full vision to no vision. Acuity is usually measured on a scale that compares a person's vision at 20 feet with that of someone who has full acuity. Therefore, a person who has 20/20 vision sees objects at 20 feet with complete clarity, but a person with 20/200 vision sees at 20 feet what a person with full acuity sees at 200 feet.
In the United states, legal blindness is defined as visual acuity worse than 20/200 in the better eye even after correction with eyeglasses or contact lenses.
Several disorders can lead to blindness including:
- cataract, which is the most common cause and can be cured with surgery
- infection, which is preventable and is not common in North America
- diabetes, which is often preventable
- macular degeneration, which affects central vision and is preventable and treatable in fewer than 10% of people
- glaucoma which is highly treatable and if treated early should not lead to blindness