Accommodation is a natural ability for a person to adjust eye focus to maintain clear images for objects at different distances. The main source of accommodation is the ability to change the focal length of your eye lens. The lens is flexible when you are young and hardens as you age. The hardening of your lenses causes presbyopia, which is the loss of ability to zoom in and see things that are small and up close clearly.
While accommodation acts like a reflex, you don't have to think about adjusting your eye's focus to switch from looking at a landscape to reading a book. It can be consciously controlled by focusing on a far away object.