The wild ancestors of the modern cat had to maximize the time they could search for food and needed a way to see in the slightest glimmer of light. Cat eyes have evolved to be super-efficient aids in order to hunt prey skillfully. This has affected the structure of their eyes in several ways.
As you have probably noticed, cat eyes are huge in comparison to the size of their head. Actually, a cat eye is almost the same size as a human eye. In darkness, their pupils expand to three times the area a human eye can. Due to being nocturnal animals, felines have a reflective layer behind the pupil known as the tapetum lucidum, which further enhances the efficiency with which their eyes capture light by up to 40%. Incoming light that misses the receptor cells in the retina bounce off the tapetum lucidum and back through the retina again. Any light that misses the second time around will pass back through the pupil, giving the cat its characteristic green eye glow whenever a light is shone into its eyes in the dark.
The receptor cells on the pupil are also arranged differently from a human’s. Receptor cells fall into the two basic types; rods for black-and-white vision in dim light, and cones for color vision when its bright. Cat eyes have mainly rods, while human eyes have mainly cones. Because their rods are connected together in bundles, they have fewer nerves traveling between their eyes and brain than ours. The advantage is that they can see in near dark; the disadvantage is that in brighter light, they miss out on finer details.
While the cone receptors in the human’s eye are centered in the retina, allowing for a more detailed picture, the small number of cones that cats do have are spread all over the retina so they get a general picture of their surroundings during the day. Because cat pupils are so large, they cannot be shrunk to a pinpoint like ours can in bright light. Interestingly, cats have evolved to narrow their pupils to a vertical slit, less than a millimeter wide protecting their sensitive retinas from light. They also half-shut their eyes, covering the bottom and top of the slit and leaving the center exposed.
Cats also show little interest in color. Similar to dogs, cats see only two colors: blue and yellow. Moreover, pattern, shape or size seems to matter more. Like humans, cats have binocular vision, but the drawback to having such large eyes is that they are not easy to focus. Humans have muscles in their eyes to distort the shape of the lens to allow close vision. Cats have to move their whole lens back and forth like a camera. Because it is cumbersome and just too much effort, most felines don’t bother to focus at all unless something exciting, like a bird flying by, catches their attention. Close focus of anything nearer than a foot away is out of the question. To compensate, cats can swing their whiskers forward to provide a 3-D tactile picture of objects that are right in front of their noses. Amazingly, the muscles that focus the lens set themselves to the environment the cat grows up in. If it is an indoor cat, it is usually more short-sighted; a cat that is outdoors is usually slightly long-sighted.
So, while the old wive’s tale that a cat can see in pitch black isn’t necessarily true, they are still pretty amazing creatures with amazing sight capabilities. As powerful as they are beautiful, it’s clear to see why humans, from the ancient Egyptians to the people of today, continue to mimic the aesthetic of the unsurpassed mystery that is the ‘cat eye’.