The Cornea-Lens-Retina System

All light rays has to pass through a focussing system that involves the cornea, the lens, and spaces in the eye filled with watery fluid.The conea and the lens are supported by the fluid, which lies between the cornea and the lens. It gives nutrients to the cornea, theat does not have any blood vessels. The fluid supports the lens and gives shape to the eye.
When light rays pass into the cornea, they are focussed right away. The cornea changes the direction on the incoming light rays, so they could be focussed and meet towards the retina. The cornea gives most of the focussing to our eyes, and the lens does the left over focussing. Most people think that the lens does all the focussing, which is not true, it is the cornea that does the most.
Lens: Lens has the ability to fine-tune our focus. It just automatically change the shape. When some certain muscles in the eye decreases, there is less tension for the lens,which allows the lens to be thicker.
Looking at distant objects, the same muscles relax, which will increase tension on the lens. This allows the lens to become thinner. When your eyes feel it's very hard to focus when you look at an object up close, and try to see it clearly.