What process occurs in the eye during accommodation?

During accommodation, the primary event is the contraction of the ciliary muscles, which leads to a change in the shape of the lens. When ciliary muscles contract, they release tension on the zonules (or suspensory ligaments) that pull on the lens. This allows the lens to become more rounded, increasing its optical power, which is essential for focusing on near objects. This process is critical as it enables clear vision at various distances, facilitating activities such as reading or looking at nearby objects.

The other options do not correctly describe the process of accommodation. For instance, the iris's role is to regulate the amount of light that enters the eye, but it does not directly influence the lens shape during accommodation. The optical nerve is responsible for transmitting visual information from the retina to the brain, but it does not adjust focus itself. Additionally, the retina does not shift position; rather, it remains fixed at the back of the eye, with accommodation involving adjustments through the lens to project the appropriate image onto the retina. Thus, the correct understanding of how the ciliary muscles interact with the lens is vital for grasping the concept of accommodation in the eye.