Accommodation
The lens has to constantly keep changing its thickness to focus the image clearly on the retina so the best image can be obtained. The image we see is inverted (turned back to front and upside down) on the back of the retina. Our brain turns the image the right way up.
When we look at near-by objects the light from the object is diverging (spreading out). This needs a strong lens to focus the light on the retina.
The ciliary muscle contracts, which causes the suspensory ligaments to slacken. This makes the lens thicken which increases its strength as a lens.
When we look at an object far away the light entering our eye is almost parallel and so does not need to be bent as much. As a result a weaker lens is now needed.
The ciliary muscle relaxes which causes the suspensory ligaments to tighten pulling the lens thinner and so weaker.
