In our youth, the natural lens in our eye is flexible and accommodates easily. As we mature, layers of protein form inside the outer lining membrane of the lens similar to the formation of rings on a tree trunk, causing the lens to become thicker, denser, and less flexible. At a certain age, close focusing becomes difficult because the lens has lost flexibility, and we need magnification to see near objects. This condition, called presbyopia, usually begins in our mid-40s and gets progressively worse for about 20 years, then stabilizes.
How Presbyopia Affects Vision
If you look through a single lens reflex camera (without using auto-focus), the barrel of the lens rotates to adjust focus. If you focus at a distance, things close up are blurry, and vice versa. The human eye works in a similar fashion. The optically perfect eye sees distant objects easily. To look at something up close, focus is adjusted by a process called accommodation. A circular muscle behind the iris contracts and moves forward slightly. This muscle is connected to the lens by tiny fibers called zonules. The change in tone and position of the ciliary muscle changes the tension of the zonules on the lens. This “tug” causes the lens to change curvature, increasing focusing power.
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