Cataract is a clouding of a natural lens of the human eye that can cause vision problems. In a normal eye, light passes through the lens and gets focused on the retina. To help produce a sharp image, the lens must remain clear. In its early stages, a cataract may not cause a vision problem and cloudiness may affect only a small part of the lens. Over time, the cataract may grow larger and cloud more of the lens, making it harder to see.