When the eye observes objects located at various distances, which...

When the eye observes objects at various distances, the object distance $u$ changes.

To form a sharp image on the retina, the eye-lens adjusts its focal length $f$ by changing its radii of curvature with the help of ciliary muscles. This process is known as accommodation.

Since the position of the retina relative to the eye-lens is fixed, the distance of the image from the eye-lens always remains constant.