Which part of the eye performs approximately three-quarters of visual focusing?

The cornea is the transparent front part of the eye and is primarily responsible for refracting light and focusing images onto the retina. It accounts for about three-quarters of the eye's total focusing power. This significant role is due to its curved shape, which bends light rays as they enter the eye, allowing for the initial focusing to occur before the light passes through the lens.

The lens, while also critical for focusing light, primarily fine-tunes the focus, allowing the eye to adjust for different distances (a process called accommodation). The iris regulates the amount of light that enters the eye but does not play a direct role in focusing. The fovea is the part of the retina responsible for sharp central vision but is not involved in the focusing process itself. Thus, the cornea's primary function in bending light rays makes it the key component in visual focusing.