Anatomy of the eye

The human eye has the sharpest vision when light rays passing through the structures of the eye meet at a sharp focal point on the foveal area of the retina. 

Visual distortion may occur although the cornea and lens of the eye may be perfectly round.  If light rays are focused before they reach the retina (a condition known as Myopia or Nearsightedness), distance vision will not be clear.   If light rays are focused behind the plane of the retina (a condition known as Hyperopia or Farsightedness) vision may or may not be clear depending on the amount of hyperopia and the amount of someone’s focusing ability.  A hyperopic person may see clearly both at distance and near, but if too much focusing effort is needed, discomfort, headaches, etc. may occur.  If not enough focusing ability is available, vision will not be clear up close or at both up close and distance.  

Distortion of vision can also occur when the curvature of the cornea is football-shaped (a condition known as Astigmatism) rather than uniformly round as light rays will not be focused at a sharp focal point.

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 reinterperets the image, making it appear right-side up.