Cataracts produce a progressive, painless loss of vision.
Because all light entering the eye must pass through the lens, any part of the lens that blocks, distorts, or diffuses light can cause poor vision. In bright light, the pupil constricts, narrowing the cone of light entering the eye, so that it can't easily pass around the cataract. Thus, bright lights are especially disturbing to many people with cataracts, who see halos around lights, glare, and scattering of light. Such problems are particularly troubling when a person moves from a dark to a brightly lit space or tries to read with a bright lamp. People with cataracts who also take glaucoma medication that constricts the pupils may have greater vision loss.