Regulatory Function of the Eyes … Focus-adjusting Mechanism

Our ability to see objects in clear focus is due to the linking of light entering the eye to a focal point on the retina.
In addition, the eyes must not simply be capable of seeing objects at a certain fixed distance, but must be able to see objects at a wide range of visual distances to enable us to adapt to various situations in our daily lives.

State when looking into the distance in focus

For example, right figure shows the state when looking at an object in focus in the distance. If the eye looks at an object nearby in precisely the same state, the focal point will be located behind the retina, resulting in a blurred image, as illustrated in the lower right figure.

Eye regulatory function

Like a camera lens, the eye has a function that enables it to vary refractivity (the ability to bend light rays) in accordance with distance to match the focal point to the retina.

This is called the "regulatory function" of the eye, a role fulfilled by a lens in the eye known as the "crystalline lens."
In cases such as that illustrated in right figure, the crystalline lens is expanded to realize strong refractivity and match the focal point to the retina, as shown in the lower right figure.

Expansion of the crystalline body

When we look at an object nearby, the muscle located in a part of the eye called "ciliary body" causes the crystalline lens to expand in order to adjust the focus, but to allow the crystalline lens to adopt a thin shape (i.e. when we look into the distance), this muscle simply relaxes, and this means that the task of looking at objects nearby is more tiring to the eye than looking into the distance.

In other words, it is easier to look at objects in the distance.
Incidentally, presbyopia, or farsightedness due to increasing age, is caused by phenomena such as hardening of the crystalline body and/or weakening of the ciliary muscle with increasing age, making it more and more difficult for the crystalline body to expand.