Cataract. Simulated view of a busy city street, as seen by someone with cataracts. A cataract is a clouding of the lens of the eye. It causes the patient's sight to become blurred and yellowed, and if left untreated can obscure the vision completely. Cataracts can be caused by diabetes or prolonged exposure to ultraviolet light, but also may be due simply to old age. It is treated by surgically removing the cloudy lens and replacing it with a plastic one. For an image showing how this scene would appear to someone with normal vision, see image C003/7146.