A. Importance of eyes - How and why we see
     After millennia of recorded history, we are just beginning to understand the complexity and diversity of eyes. Most living creatures and some plants have individually designed eyes. Sight is essential for most creatures. For most of us the eye provides the most important link to the world by enabling us to visualize shapes and colors. Some animals and plants only sense changes in light without seeing specific images. The eye and brain process visual information to link our inner being to the world and beyond. 

1. How we see
     Eyes are adaptable visual sensors that enable us to see in a wide variety of situations. While moving, images must be collected without movements that cause blurred images. The electro-mechanical parts of eyes near and around the lens provide a means of tracking, adjusting light, and focusing. Tracking capability to control eye direction and focus is required for the eyes of most creatures. Here the brain controls where and how eyes see as well, as what they see. Brain image processing software and hardware for eyes are more amazing than the eyes themselves.
     Scientists are just starting to understand the complete process of vision. The following is a diagram of a typical man-made image display system (Figure 1.1).
     As an example of how we see, consider the current generation of focal plane array sensors such as charge-coupled devices in typical video cameras. They are also used
for military and commercial infrared surveillance or visible viewing systems, optical missile warning systems, and automatic optical target recognition systems. These systems are much like our eyes. Some vision system optics and light control appear to be patterned after a human eye as shown in Figure 1.2. (From p. 135, Iridology, Vol. 2, 1982, published by Bernard Jensen Enterprises,  Escondido, CA 92027)

Eye Design Book
Dan-Eric Nilsson, Animal eyes. Editeur : Oxford University Press (29 novembre 2001)