Unlike other refractive errors such as nearsightedness (myopia), farsightedness (hyperopia) and astigmatism, presbyopia is caused by an age-related process that leads to the eye’s lens losing its flexibility.

It is generally believed that as we age, changes in the lens’s proteins make the lens more rigid and less flexible over time. Also, muscles surrounding the lens may lose their elasticity. As the lens becomes less flexible and able to change shape as easily as it used to, the eye has a harder time focusing on close objects. This is why people over age 40 often find themselves holding reading material farther away to be able to see it clearly.