Since I recently got my first dog-Sadie a Irish Setter, I have been wondering why she isn’t seeing things like a squirrel in the tree overhead. So, I have been doing a little research.
A dog’s eye functions much the same as any mammal’s eye. The eyeball is roundish with the nerve filled membrane, called the retina, lining the back of the eyeball. Light is focused, and information is transmitted to the brain via the optic nerve. The dog’s eye has a reflecting layer, called the tapetum lucidum, which intensifies available light, giving the dog an advantage during dusk or dawn, the prime time for hunting.
Because dogs have two eyes, they have binocular vision. Binocular vision is the area within a dog’s total vision field that overlaps, providing it with depth perception. The exact degree of binocular vision within a dog’s total visual field depends on the shape of the dog’s head and the exact placement of the eyes.
Most dogs have a total visual field of around 250 degrees. The degree of binocular overlap is about 75 degrees for long-nosed dogs to 85 degrees for short nosed breeds. Humans have about 120 degrees of binocular vision, but since their eyes are set directly on the front of the face a human’s total visual field is only around 190 degrees, giving dogs the advantage of 60 degrees more peripheral vision.
Although dogs have greater peripheral vision, they cannot perceive detail as well as humans. Dogs are more in the 20/75 acuity range as oppose to human’s 20/20. This means that the visual acuity that a dog sees at 20 feet is similar to what a human would see at 75 feet. Like big cats, objects that are stationary can elude their notice. When undecided about what they are seeing, dogs depend on their sense of smell to confirm any doubts. Although motionless objects can be missed, a dog’s sight is very sensitive to moving objects. They can perceive direction, speed and may even be able to recognize an animal or human by their pattern of movement.
Dogs were once believed to be color blind, but scientists now agree that dogs have enough color preceptor cones in their eyes to perceive a limited palate of colors. Like people, dogs have these photoreceptors in their retinas, but they’re not as highly adapted, so their color spectrum is not vivid or broad. Dogs don’t see as much red and green as we do, but they can see lots of blues, yellows and grays.
Dogs, like people, age and succumb to disease. Dogs can get glaucoma, dry eye, corneal ulcers, retinal disease, and retinal detachment. Dogs are less likely to get infectious eye diseases.
dog owners should look for these signs to check if their pet’s vision has deteriorated: hesitating in dim or dark light, difficulty navigating stairs, discomfort in bright light, dilated pupils, tearing or squinting, irritated red eye, cloudy eyes, excessive discharge, and eye pain.
Have fun this summer!
Torrey Carlson, O.D.