Of all the cat's senses the eyes rank first in importance. The cat relies on its eyes to hunt and capture prey. Their eyes must be able to function in a wide range of conditions from bright light to near total darkness.
Cat's eyes are very large in relation to their body size. For instance, our common domestic cat has eyes just slightly smaller than a human's eyes. Large eyes with large pupils have great light gathering capabilities. In bright light the pupil of a cat's eye closes down into a mere slit, while in darkness, the pupil dilates into a large round disk almost taking up the whole space of the eye.
In mammal's eyes there are two types of light sensitive cells. Rods function in low levels of light while cones are used in color vision. Rods predominate in the cats eye. However, cats also have a cone rich patch in the center of the retina which means that cats can see color. Studies indicate that cats can see green, blue and even red. Present data suggest that cats do not experience the saturation of colors that humans can see.
The cat's sensitivity to light is also heightened by a structure known as the tapetum lucidum, a special reflective layer beneath the retina. Caught in the bright beam of a flashlight, a cat's eyes shine back with a yellowish green light. This "eye shine" as it is called comes from the tapetum. The mirror like tapetum reflects the light back through the retina to give the sensory cells a second chance to respond. It has been estimated that a cat's light sensitivity is nearly six times that of human.
Cats have the most highly developed binocular vision of all the carnivores. Their eyes are set well forward and relatively high on the skull. This allows the cat to accurately judge distances while leaping from branch to branch or pouncing on prey. Cats also have an extensive field of peripheral vision that allows the resting cat to focus his eyes infrequently. This is recognized as the wide eyed stare off into space look.