There are multiple parts of the eye that help to bend the light and make the image on the retina. One of these elements is right behind the visible pupil or iris. It is called the lens but, confusingly, it is just one of the parts that is helping to bend the light into focus. It is important for its ability to change shape though.
When the lens gets pulled on by some small muscles it changes shape from more curved to less curved. This variation in shape changes the focusing distance of the whole eye assembly. A stretched, less round, lens causes targets that are far away to be in focus while a relaxed, more bulbous, lens leads to things closer to the eye being in focus.
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