Centuries of domestic breeding have resulted in cats and dogs that come in a wide variety of shapes, sizes, and colors. If you have a favorite breed, there’s a good chance that you like it to some degree because of the way it looks. But what do our pets see when they look back at us?
Let’s clear up the most common misconception first. Dogs do not see in black and white. They do, however, see a different color spectrum. This is because their retinas—the light-detecting membranes at the back of the eyes—are built differently. Human retinas have three types of light-sensitive cells called cones, each of which is tuned to a single color: red, blue, or green. Dogs have only red and blue cones, which makes their vision similar to that of a person with red-green color blindness.
Like humans, cats have three types of cones, but they still don’t see color all that well. This is because cats and dogs have another problem with color vision: Regardless of which cones they have, they don’t have very many of them. Instead, their retinas are packed with a different kind of light-sensing cells, called rods, that don’t detect color at all. Rods are better suited to seeing in dim light than they are to parsing the hues of rainbow. People have fewer rods than cones, so while we get to see the daytime world in bright color, we are fated to stub our toes searching for the toilet at night.
But all those rods aren’t the only reason why cats and dogs can see so well in the dark. You’ve likely noticed your pets’ eyes glow bright green at night. This is courtesy of the tapetum lucidum, a reflective layer behind the retina. Any light that slips through the retina bounces off this secondary layer for another pass through the animal’s retina, effectively doubling its sensitivity.
There’s more to vision than color and brightness, however. Compared to people, dogs and cats have limited visual acuity. Dogs have roughly the equivalent of 20/75 vision, meaning they need to be 20 feet away from something to see it as well as a normal person could at 75 feet. And you may be surprised to hear that cats fare even worse! Those sleek and gorgeous eyes seem built for precision, but cats are close to legally blind with vision somewhere around 20/150!
Making matters worse, dogs and cats have trouble adjusting their vision to different distances. This is because their lenses can’t adjust shape as readily as ours can. If you’re over 40, you’re familiar with what happens when your lenses start to become inflexible. It gets harder and harder to focus on anything close to your face. Welcome to life as a dog.
The short of it is that cats and dogs see better at night than we do, but those adaptations come at the cost of clarity. But poor vision doesn’t slow them down any. They don’t need to drive cars or read the fine print. And what they lack in eyesight, they make up with magnificently superior senses of smell and hearing. Even animals that lose their vision due to degenerative diseases do incredible job of navigating their homes, because their vision was never that great to begin with.
Dr. Mike Fietz is a small animal veterinarian at Georgetown Veterinary Hospital. He moved to Charlottesville in 2003, the same year he received his veterinary degree from Cornell University.