While cats may reign as the pet world's champions of night vision, dog lovers can rejoice: Their beloved canines can also navigate in dim light, if not quite as skillfully as their feline companions. How can dogs get by when the lights go out, and on a 20/20 scale, can canines go eye-to-eye with humans?
Dogs' ability to see in darkness stems from the structure of their eyes. They have large pupils, which let in more light. Their retinas have a lot of light-and-motion-sensitive cells, called rods. These distinguish light from shadow. And most importantly, dogs' eyes, like those of cats, some birds and fish, and nocturnal predator animals, have a mirror-like membrane called a tapetum lucidum at the rear of the eye. The tapetum allows light that is not absorbed by the rods to rebound to the retina, letting it take in more light, which enables the dog to see better.
But do dogs see better than humans? The simple answer is that they see differently, and in some instances, better. Dogs see shades of gray, while a human's world is in living color. The human retina has more cone cells, which distinguish color, while dogs' retinas have more rods, which need much less illumination to detect the gray spectrum. In twilight or indoor semi-darkness, a dog can see more clearly than his human. The rods are also sensitive to motion, allowing dogs to detect smaller movements and to quickly sense a stranger, or their prey. This is helpful when hunting at night or on guard duty.