The part of the eye that changes shape to focus light is the lens. It is a transparent, biconvex structure that bends incoming light so it forms a sharp image on the retina.
This focusing process is called accommodation. In humans, the ciliary muscle changes tension on the suspensory ligaments around the lens. For close objects, the lens becomes rounder and more strongly refracts light; for distant objects, it becomes flatter. The cornea also bends light, but it does not normally change shape to perform this focusing adjustment.
The lens grows throughout life by adding layers of fiber cells. With age, it gradually becomes less flexible, making near focusing harder; this condition is called presbyopia. Clouding of the lens causes a cataract, while the iris controls how much light enters the eye and the retina detects the focused image.