By Diana Bocco
Dogs don't see the same world we do. For them, things might look a little grainier and a lot less vibrant. Then again, dogs can also see things we can only dream about.
Here are six fascinating facts about your dog's vision.
While scientists used to believe that dogs were color blind, turns out your dog can see colors, but with a different spectrum. "Dogs do have fewer color sensitive cone receptors in their retinas than their human counterparts," says Dr. Martin Goldstein, an integrative veterinarian and author of The Nature of Healing Animals. "In essence, this would make them similar to a person who is red-green color blind."
That doesn't mean dogs can't see red—it just doesn't look as vibrant to them. "That bright red toy has always looked a brown shade to them," says Dr. Gary Ryder, who works for VCA Southwest Michigan Animal Emergency and Referral Center. "This is why you may see a preferential treatment toward the blues and violets, as that is where the strength of their cones lies."
On the other hand, dogs' vision is more in tune with sepias and pastels than the full range that humans see, according to Ryder. "It has actually been reported that they can very accurately differentiate among different shades of gray, even though they would appear closely associated to us humans," Goldstein adds.
When it comes to sharpness of vision, dogs don’t fare as well as humans. A person with 20/20 vision can see what the average individual can see on an eye chart when he is standing 20 feet away. "When it comes to dogs, they are more in the 20/75 range,” Ryder explains. "This means that the visual acuity that a dog sees at 20 feet is similar to what a human would see at 75 feet."
In human terms, the average dog would be considered somewhat nearsighted, Goldstein says. Think of it in terms of pixels, Ryder suggests. "If you remove 75 percent of the pixels in a normal clear image, that is what a dog would be seeing," Ryder says. "It's still clear, it's just more grainy than what we see."
The field of view of the canine eye is usually 240 degrees, according to Ryder. "This is better than humans (180 degrees) and cats (200 degrees)," he says. However, this varies greatly between individual dogs and breeds.
"The wolf retina is the most sensitive to scanning a horizon and looking for predators/ prey," Ryder says. "A brachycephalic breed (Bulldogs, Pugs, Boxers, Boston Terriers) with forward-sitting eyes will see to the periphery better but they cannot see as well right in front of them."
While you probably already knew that cats can see really well in low light, the truth is that dogs can see almost as well as their feline friends. "Dogs and cats can see about seven times better in low or dim lights than people," Ryder says.
This is in part due to something called a tapetum at the back of the retina. "This is a reflective layer so light that is not absorbed by photoreceptors gets bounced back and forth in the back of the eye to give it another chance at being recognized," Ryder says. "Cats are especially good at this and they reflect 130 times more light than humans. This tapetum is what gives the glowing-eye look when you see a cat in the dark; that glow is light that is bouncing off the tapetum and then through the pupil and back to your eye."
While the third eyelid is an important part of your dog's eye, it's not technically used in vision.
The third eyelid (officially called the nictitating membrane) has a couple of functions. It is there, primarily, to protect the eye. "There is a very sensitive reflex called corneal reflex where any sensation on the cornea makes the third eyelid go up, outer eyelids close, and a muscle behind the globe pull the eye back," Ryder explains. The third eyelid also functions to produce tears, he adds.
One of the easiest health issues to detect through the eyes is liver disease. "With severe liver disease, a secondary condition occurs where there is retention of bile," Goldstein says. "This is commonly known as jaundice and probably the easiest place to detect this condition is a yellowing of the whites of the eyes."
On the other hand, a very pale sclera (the white outer layer of the eye) can be a sign of anemia from an abdominal bleed or kidney disease, according to Ryder.
In dogs, changes in the size of the pupil can also indicate a serious medical problem. "The pupils can be small when there is pain in the eye," Ryder says. "And two different sized pupils can be present with head trauma." The presence of blood in the space between the cornea and pupil can also indicate trauma or potentially rodenticide poisoning (rat bait).
A cloudy eye can indicate the presence of protein or white blood cells. "This can be secondary to trauma, infection, auto-immune disease, cancer (lymphoma especially), diabetes, and many other conditions," Ryder says.