The change in the focal length of an eye lens is caused by the action of the Ciliary muscles.
The ciliary muscle is a circular muscle that relaxes or tightens the zonules to enable the lens to change shape for focusing. The zonules are fibres that hold the lens suspended in position and enable it to change shape during accommodation. It increases or decreases the focal length of the eye lens. The ciliary muscle contraction loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision.