The optical system of the eye is similar to a camera that uses film.
Images focus through a lens projecting a clear internal image. The physiologic difference between the eye and a camera is the human lens degrades with age resulting in unfocused, decolorized and generally cloudy vision.
This optical deterioration of the lens is due to cataract formation.
Age related cataracts are the most common but cataracts can develop after injuries or use of corticosteroid drugs. Health conditions such as diabetes, lupus and uveitis can be associated with premature cataract development while on some occasions babies are born with cataracts.
Eyeglasses or contact lenses can sometimes help improve vision caused by cataracts, will not improve vision if a severe cataract is present. In such instances cataract surgery is required.