How does the focal length of the eye lens change when we shift looking from a nearby object to a distant object ?

It is made possible because the focal length of the human lens can change i.e., increase or decrease, depending on the distance of objects. It is the ciliary muscles that can modify the curvature of the lens to change its focal length.

To see a distant object clearly, the focal length of the lens should be larger. For this, the ciliary muscles relax to decrease the curvature and thereby increase the focal length of the lens. Hence, the lens becomes thin. This enables you to see the distant object clearly.

To see the nearby objects clearly, the focal length of the lens should be shorter. For this, the ciliary muscles contract to increase the curvature and thereby decrease the focal length of the lens. Hence, the lens becomes thick. This enables you to see the nearby objects clearly.

The ability of the eye lens to adjust its focal length accordingly as the object distances is called power of accommodation.