Ever wonder what your dog or cat sees as he or she gazes upon their world? Do they see colors and, if so, which ones? Can they see great distances or only close up? What about night vision?

The answers to some of these questions might surprise you but, by knowing what your pet’s vision should be, this information may guide you and help you recognize if it is having vision difficulties.

Human versus canine

We have a good idea of what dogs and cats see, because we know the make-up of their retinas. The retina, which lines the inside of the eye, is made up of light-sensitive cells called rods and cones. Cones provide color perception and detailed vision; rods detect motion and vision in dim light.

Dogs have 10 times more rods than humans and, therefore, can see better in the dark and can detect subtle movements better than humans. Enhancing their night vision is a thin reflective tissue behind the retina called the tapetum lucidum. This tissue reflects light back through the retina, giving the rods and cones a second opportunity to absorb the light. We have all seen the tapedum in action: It’s the bright green to yellow reflection seen in a dog’s eyes when a bright light shines on them at night.

Also because of the large number of rods in their retinas, dogs see moving objects much better than stationary items; in a study, dogs could recognize moving objects more than 2,500 feet away but could recognize the same object, when stationary, at less than 1,500 feet. This motion-detecting characteristic offers a clear advantage for canines stalking prey, especially at twilight, or guarding territory against intruders at night.

Dr. Ralph Hamor, a veterinarian specializing in ophthalmology, said: “So much of dog behavior deals with posture and appropriateness. Small changes in your body posture mean a lot to your dog.” They see and interpret body language of other animals and humans, and are far more skilled than we are at this form of nonverbal communication.

Color-blind canines

The trade-off in the large ratio of rods to cones in a dog’s retina is dogs can’t distinguish the full spectrum of colors and probably see colors similar to a red-green color-blind human. Dr. C.J. Miller at the University of Wisconsin School of Veterinary Medicine reports dogs have two types of cones in their retinas: One that is maximally sensitive to violet-blue wavelengths and one that is sensitive to yellow-green light.

As a result, the canine spectrum may be divided into two hues – blue and yellow – with shades of white and gray where we can detect other colors.

So how does a guide dog determine that a traffic light is green or red to decide when it’s safe or not safe to cross a street? It can’t, based solely on the color; however, other clues, such as the position of the illumination on the stop light, the relative brightness, the sound of the traffic and reactions of other pedestrians help the guide dog keep its master safe. Though their world may not be as colorful, dogs reportedly have the ability to distinguish between closely related shades of grey that are indistinguishable to the human eye.

Pets versus people

Visual acuity is the ability to see the details of an object separately and unblurred. For humans, the ratio of 20/20 is used for someone with normal vision. This means the person being tested can determine the details of an image, e.g., letters on a chart, from 20 feet away that a normal person could differentiate from 20 feet away.

When this same scheme is applied to animals, Dr. Miller reports the visual acuity of a dog is about 20/75 and a cat is 20/100 to 20/200. This means from 20 feet away, normal dogs can distinguish the details of an object that a person with normal vision can differentiate from 75 feet away. So humans have 3 to 4 times better visual acuity than dogs in bright light, and 5 to 10 times better visual acuity than cats in bright light. However, before you begin feeling smug, remember our pets’ enhanced sense of smell, hearing, texture and taste, as well as the increased vision in dim light, more than make up for this difference.

Compared with humans’ visual system, the visual system of dogs and cats could be considered inferior in such aspects as color perceptions or visual acuity. But they are superior to us in their ability to function in dim light, to detect shades of gray and detect motion. The vision of our pets has developed to suit their needs. Owners should be on the lookout for changes in the health of their pets’ eyes, such as discharge, squinting, redness or cloudiness, and quickly consult with the pet’s doctor when these warning signs arise.

Dr. Dennis Selig is a veterinarian at Northwood Hills Animal Hospital in Gulfport, Miss.