Your eyes

The normal eye works very much like a camera: the cornea and the lens of the eye focus the image of a distant object on the retina which captures the picture just as the film of the camera. The cornea provides most of the focusing power and cannot change its shape. Small muscles can slightly change the shape of the lens; this allows us to focus up close while reading.

These muscles weaken with age this is why after the age of 43 approximately, people wearing glasses for distance need bifocals or progressive lenses to read, even people with perfect distance vision eventually need reading glasses.