The situation that results in the lens of the eye having a more rounded shape is the contraction of your ciliary muscles that relax the suspensory ligaments which support the lens and allow it to become more spherical and increase it's refractive power.
This process is called accommodation and the eye lens becomes more rounded and thicker when focusing on near objects.
Astigmatism also makes the shape of your eye more curved that it should be and causes blurred vision at all distances.
And in a condition called keratoconus, the cornea thins and then gradually bulges outwards into a cone shape and can also cause blurry and distorted vision.
The shape of your eye lens changes as a result of the contraction and relaxation of the ciliary muscles.
The ciliary muscles are attached to your suspenzory zonules which in turn connect to the eye lens.
And when the ciliary muscles contract, the zonules relax and allow the lens of the eye to become more rounded and increase it's refractive power.
And when the ciliary muscles relax, the zonules tighten, pulling the lens of the eye flatter and decreases it's refractive power.
The process is called accommodation and allows your eye to focus on objects at different distances.