A person can see objects put at different places by changing the focal length…

The human eye has the ability to focus clearly on objects located at different distances. This ability is called accommodation.

Accommodation is achieved by the ciliary muscles, which are attached to the eye lens through suspensory ligaments.

-When viewing a near object, the ciliary muscles contract, causing the lens to become thicker and more convex. This decreases the focal length.
-When viewing a distant object, the ciliary muscles relax, causing the lens to become thinner and flatter. This increases the focal length.

Thus, the ciliary muscles regulate the focal length of the eye lens, enabling clear vision of objects at varying distances.