The Human Eye

Anatomy of the eye
A human eye gives the sharpest vision when it is perfectly spherical, that is, round like a basketball. Distortion occurs when the eyeball is too short from front to back (Hyperopia) or too long (Myopia), or the curvature of the cornea is irregular, egg shaped rather than a perfect sphere (astigmatism).    

How the eye works
Light from object A is focused by the cornea and crystalline lens to form an upside-down image on the light-sensitive cells of the retina. Information is transmitted as impulses from these cells to the brain via the optic nerve. The quantity of light entering the eye is regulated by the iris. 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-interperets the image, making it appear right-side up.