The ability
to change focus from distant to close objects comes from the fact that
the eyelens is flexible. You change focus by changing the shape of
the eyelens which changes its focal point.
The process
of changing the lens' shape is done by ciliary muscles. When those
muscles
are relaxed, the eyelens is pulled on by suspensory ligaments. This
causes the focal point to be far away which allows you to focus on far
away objects.
To see objects
close to you, ciliary muscles become tense, forming a smaller inner ring,
which allows for ligaments to relax causing the eyelens to bulge into
the shape
it prefers.
Great focusing precision is achieved by adjusting the weaker element
slightly (the eyelens) rather than adjusting the strong focusing
element (cornea).
The strain you feel after many hours of close up work (reading, writing, etc.) is due to the fatigue of the ciliary muscles.
Normal eyes are able to focus comfortably between infinity and 25cm