A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. Cataracts are the most common cause of vision loss in people over age 40. Today, cataracts affect more than 22 million Americans age 40 and older. The lens inside the eye works much like a camera lens, focusing light onto the retina for clear vision. It also adjusts the eye's focus, letting us see things clearly both up close and far away. The lens is mostly made of water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it. As we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract, and over time, it may grow larger and cloud more of the lens, making it harder to see.