A dog's eye functions much the same as any mammalian eye. The eyeball is round in shape with a light sensitive membrane, called the retina, lining the rear of the eyeball. Incoming 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 the depth of perception needed to pursue prey. 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 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 (and not the side of the head) a human's total visual field is only 190 degrees, giving dogs the advantage of 60 degrees more peripheral vision. There are dog breeds, such as the Chow Chow, however, that have such deep set eyes that their peripherial vision is reduced-- a factor that should be kept in mind when approaching such breeds from the rear.
Although dogs have greater peripheral vision, they cannot perceive detail as well as humans. 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.
Most dogs have brown eyes, but there are breeds with pale blue, speckled, golden or hazel colored eyes. Some dogs are odd-eyed, having one eye that is blue and another eye brown. The shape of the eye and its placement on the head varies with different breeds. Most are oval and placed midway between the side and front of their faces.
Dog fanciers have terms to describe certain eye colors and shapes:
· An eye that is clear blue but flecked with a white or lighter blue is known as a China Eye.
· Dogs with a prominent, visible third eyelid (nictitating membrane) are said to have Haw Eyes. Haw eyes are seen in such breeds as the St. Bernard and Bloodhound.
· Triangular eyes have a three cornered, tent shaped appearance and are seen in Afghan Hounds.
· Wall eyes, characterized by a pale bluish-white iris with flecks of brown, are seen in some Harlequin Great Danes.
· Prominent eyes are big, round projecting eyes such as seen on Pugs.
· Other eye shapes include Almond, Circular and Oval.