Which transparent structure in the eye changes shape to focus on objects at different distances?

The lens

The lens adjusts its shape through a process called accommodation, using ciliary muscles to become thicker or flatter depending on an object's distance. This flexibility allows the eye to maintain a sharp focus on the retina, acting much like the internal autofocus system of a camera. As people age, the lens naturally loses this elasticity, which is why many eventually require reading glasses to see close-up.