The muscles of a normal eye are least strained when the eye is focussed on an object
Human eye lens has fibrous, jelly-like materials
Ciliary muscles change the curvature of the eye lens depending upon the distance of the object
that changes the focal length.
When the object is close to eye, the contraction required is more since the curvature of eye lens
has to change, and when the object is distance, less contraction of ciliary muscles takes place.