Light from object A is focused by the cornea and crystalline lens to form an upside-down image on the light-sensitive photoreceptors of the retina.  Information is transmitted from these photoreceptors to the brain via the optic nerve.  The quantity of light entering the eye is regulated by the iris (colored part of the eye).  The pupil is the variable-sized black circular opening in the center of the iris.

This is an animated example of normal spherical vision.
Please note: The brain re-interprets the image, making it appear right-side up.