How does your eye refocus from a distant tree to the page in your hand?

Your eye refocuses by changing the shape of a small, clear, elastic lens, letting it round up or pulling it flatter. Looking at something close, a muscle around the lens tightens and the lens bulges. Looking far away, the muscle relaxes and the lens is pulled thinner and flatter. This tuning is called accommodation, and it happens so automatically that you never notice it.

The lens is not doing the heavy lifting, though. The cornea, the clear dome at the very front of the eye, supplies about two-thirds of the eye's total focusing power. But its curve is fixed. It is a lens you can't adjust. The crystalline lens behind the pupil adds the remaining third, and it is the part that can change, so it handles the fine focusing.

Here is how the tuning works. The lens hangs from the ciliary muscle by fine fibres called suspensory ligaments. When you look at something near, the ciliary muscle contracts, which eases the tension on those fibres and lets the springy lens relax into a more rounded shape that bends light harder. For a faraway object the muscle relaxes, the fibres pull the lens thinner and less curved, and the light is bent less.

The design has a human story. In 1800 Thomas Young proposed that the lens changes shape, and he even wondered whether the lens was a muscle itself. He went looking for the nerves that would make it squeeze, and never found them. Helmholtz and Huxley later refined the idea into the version taught today.

The system wears out. After about 40, the lens stiffens and the muscle weakens, so near objects stay blurry. That is why so many adults end up holding menus at arm's length, or buying reading glasses with convex lenses.