Cataract. Artwork of two eyes, one with a cataract (right). Cataract is a condition in which the lens of the eye loses its opacity. The lens of the left-hand eye is black, but the right-hand lens has become cloudy and is cream. Cataracts are a natural feature of ageing, but can also be caused by injury to the eye or certain diseases, such as diabetes. It leads to progressive loss of vision and may result in blindness. Treatment is with surgical removal of the lens and replacement with a plastic artificial lens, and is very successful. Also seen is the sclera (white of the eye), iris (light blue), pupil (dark blue) and ciliary body (red).