Puppy and adult dog eyes work quite similar to your own. Light passes through the clear window-like cornea on the front surface of the eye, through the dark round opening called the pupil, and enters the lens which focuses the light images onto the retina at the back of the eye. Unlike human eyes, all dogs have a "third eyelid" called the haw or nictitating membrane which is located in the inner corner of the eye that acts as a windshield wiper that sweeps horizontally across the eye.
What Dogs See
“Most dogs are emmetropic—normal sighted,” says Christopher Pirie, DVM, a veterinarian certified in ophthalmology and an Assistant Professor at Cummings School of Veterinary Medicine at Tufts University. He notes that they tend to be very slightly nearsighted when compared to human, with an average refractive error of -0.05 D.
Dogs tend to rely more on motion than focus, though. They have trouble seeing objects closer than about 10 inches and must use their noses to find the kibble that spills out of the bowl.
Good vision can be important especially for service animals that need to pay very close attention to human partners, or run interference as protection dogs. Poor vision may interfere with competition or hunting dogs seeing the handler’s directions. But contacts aren’t practical when dogs lose them so easily.
Dogs do benefit from being fitted with glasses. A veterinary ophthalmologist evaluates vision by refraction in the same way non-verbal children are examined. Products like Doggles are designed to fit the canine face in all its various shapes and sizes. They can be used to protect sensitive eyes from injury or include corrective lenses to improve canine acuity.
Can Dogs See Color?
Can your puppy tell the difference between that red ball and the blue one? The retina at the back of the eyeball contains specialized cells called rods that detect shades of white, black and gray while cone cells detect color.
Dogs have fewer cone cells than people do and see a dichromatic—or two-color system. Dr. Pirie says the dog’s cones are most sensitive to deep blue and green wavelengths. In comparison, people see a three-color system of red/yellow/blue. Dogs seem to be similar to people who are “color-blind” and unable to detect certain colors like red.
Dogs can be easily trained to tell the difference between certain colors even though they don’t see them in the same way as people. Under normal light, dogs probably see green and blue as much brighter than red, because they have very few to no red-sensitive cones.
Like human eyes, the dog's eye can control the amount of light that passes through the eye. The specialized muscle called the iris (the colored portion of the eye) can contract the pupil to a round pinpoint in bright light, or open wide to allow in more light during low light.
The lack of color sense is balanced by the dog having many more rods, the light-sensitive cells on the retina, than people do. The retinal illumination of the dog is about three times more efficient than yours.
Dog Eye Shine at Night
Dog eyes also have the ability to “re-use” existing light to improve their vision in low-light environments. Many mammals, including cats and dogs, have a layer of highly reflective cells behind the retina that reflects back any light the eye captures. The tapetum lucidum enhances the light-gathering efficiency of your dog’s eyes by nearly 40 percent and accounts for that eerie eye-glow you see at night.
“The dog’s tapetum allows for an overall lower amount of light required to generate a signal and to detect a change,” says Dr. Pirie. “The rods are more sensitive to changes in light and useful for motion detection but do not allow dogs to see well. The cones are primarily responsible for visual acuity but are not functioning at such a low level of light.”
A dog’s eyes are more sensitive to low light conditions and have a better ability to perceive changes in motion than people. But the dog’s visual acuity (how clearly they see) isn’t particularly good under these conditions.
Prey animal like rabbits and deer can watch in two directions at once with eyes on each side of the head. But predators, such as dogs and cats, have eyes toward the front of the face that gives them depth perception and binocular vision so they can correctly time pursuit and pounce.
Eyes placed closer together have a greater degree of visual overlap, improved binocular vision, compared to those placed farther apart. The binocular vision and field of vision vary somewhat between dog breeds depending on the formation of the face.
The eye placement of brachycephalic dogs like pugs is situated more toward the front of the face, while the narrow-headed sight hound breeds like the collie tend to have eyes more on the sides of the face. Most dogs have only about 30 to 60 degrees of binocular overlap versus approximately 140 degrees of humans. Therefore the depth perception by dogs is not as acute as for people. The dog may see the movement of hand signals from a long distance but not recognize a hat-wearing owner until scent or voice further identifies you.
Dogs are champions when it comes to the visual field of view, seeing all around in peripheral vision. That means when your puppy looks straight ahead it can still see 240 degrees, compared to 180 degrees in humans.
Dogs are potentially even better at peripheral vision because they have a high-density line of vision cells across the retina, called a visual streak. That lets them see a sharply focused object at a distance, even in the extremes of peripheral vision, out of the corners of their eyes.
“It is believed this extension of cones improves the animal’s ability to see particularly along the horizon and serves as an adaptation based on the evolutionary requirement of the animal,” says Dr. Pirie.
A reliance on motion makes sense from an evolutionary standpoint. The dog breeds developed to hunt by sight like greyhounds and Afghan hounds scan the distance for prey. Most dogs can detect strong hand signals from as far away as a mile.
While dogs tend to ignore stationary objects, this visual streak triggers their instinctive urge to chase whenever something moves in their peripheral vision. That makes dogs great guards because they alert to even small movements humans probably would never see. The visual streak is most pronounced in long-nosed dogs such as the sight hound dogs developed to hunt and chase. That’s why herding breeds, for instance, may trigger on a bicycle or car and turn into chase maniacs.
Can Dogs See TV Images?
Many of the short-nosed dogs like pugs don’t have this visual streak. Instead, they have high-density vision cells arranged in a single spot on the retina, called the area centralis. The area centralis has three times the density of nerve endings as the visual streak. While Dr. Pirie says it’s difficult to truly assess these differences, some researchers report this may be why short-nosed dogs seem to react to the television screen or perhaps appear more sensitive to human facial expressions.
This may be why smaller breeds are cherished not only fit better on the owner’s lap or for hunting or guardian abilities. Dogs able to see well in near-vision (lap-to-face) distances have the physical ability to be more responsive to your moods and emotions.