Cataract. Close-up of a congenital (present from birth) cataract in a patient's eye. It is seen as the milky circle within the black pupil. A cataract is a progressive opacity of the lens of the eye due to changes in its structure. It causes blurred vision that can progress to blindness if it is untreated. Cataracts are very common in the elderly, and are considered a part of the aging process. They can also be caused by an eye injury, diabetes, some drugs and exposure to radiation. Surgical treatment can restore vision. The lens is removed and a plastic replacement inserted.