What happens when we want to focus on a nearby object, rather than one ``infinitely far'' away? Without any changes to the optical system, the image would be blurry on the retina, as shown in Figure 4.27. Fortunately, and miraculously, the lens changes its diopter to accommodate the closer distance. This process is appropriately called accommodation, as is depicted in Figure 4.28. The diopter change is effected through muscles that pull on the lens to change its shape. In young children, the lens can increase its power by an additional $ 15$ to $ 20$D, which explains why a child might hold something right in front of your face and expect you to focus on it; they can! At $ 20$D, this corresponds to focusing on an object that is only $ 5$cm from the cornea. Young adults already lose this ability and can accommodate up to about $ 10$D. Thus, with normal vision they can read a book down to a distance of about $ 10$cm (with some eye strain). Once adults reach $ 50$ years old, little or no accommodation ability remains. This condition is called presbyopia. Figure 4.29 shows the most common treatment, which is to place reading glasses in front of the eye.

Steven M LaValle 2020-01-06