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The lens is a transparent structure located behind the iris and the pupil in the eye that plays a crucial role in focusing light onto the retina. When light enters the eye, it first passes through the cornea, where it is bent slightly. However, the lens fine-tunes this focusing process by changing its shape through a process called accommodation. This adjustment allows the eye to focus on objects at varying distances, ensuring that the light rays converge precisely on the retina, where visual information is processed. The clarity and proper alignment provided by the lens are essential for sharp vision.

The other structures mentioned have important functions but do not primarily focus light onto the retina in the same direct manner as the lens. For example, the cornea provides most of the eye's total refractive power but does not have the same capacity for adjustment as the lens. The iris controls the amount of light entering the eye but does not focus it, while the vitreous body maintains the eye's shape and holds the retina in place but does not contribute to the focusing of light onto the retina.