Cataract a common cause of gradual vision loss is opacity of the lens or the lens capsule of the eye. The clouded lens blocks light shining through the cornea. This block, in turn, blurs the image cast onto the retina. As a result, the brain interprets a hazy image. Cataracts commonly affect both eyes, but each cataract progresses independently. Exceptions are traumatic cataracts, which are usually unilateral, and congenital cataracts, which may remain stationary. Cataracts are most prevalent in people older than age 70. Surgery restores vision in about 95% of patients.