The wild canines that our pet dogs evolved from are "crepuscular," meaning they are usually active at dusk and dawn. This means that they needed eyes that worked well in dim light, which means that their eyes are different than those of people in some ways.
Understanding eyes is a bit easier if you think of a camera. Both your eye and a camera require a hole to let light in (the shutter aperture in the camera and the pupil in the eye), a lens to gather and focus the light, and some kind of sensitive surface to register the image (the film or photo detecting layer in the camera and the retina in the eye). Both eyes and cameras need features to allow them to adjust to various light conditions and both are continually making compromises between working well at low levels of light or being able to see small details. At every stage in the construction of the dog's eye, the choice seems to have been made to sacrifice some ability to see small or fine aspects of the environment in order to be able to function better at low light levels.
When it comes to letting light into the eye, your dog's pupils are much larger than most humans. In some dogs you can't really see much of anything except the wide pupil filling the eye with just a hint of colored iris around the edge. The dog also has more light-gathering power in its eyes than people because of larger lenses.
To gather a lot of light, a lens has to be big, which is why astronomical telescopes, such as that at Mount Palomar in California, can have lenses as large as 200 inches (500cm) across. There are effectively two parts of the eye that serve as lenses in humans and dogs. The first is the cornea, which is the transparent portion of the eye that bulges out at the front. The cornea is responsible for the actual light gathering. The second, the crystalline lens, is behind the pupil and is responsible for changing the focus of the light. Animals that are active in dim light usually have large corneas. Notice how large your dog's corneas are in comparison to those of people. This larger size permits more light to be gathered and sent into the eye for processing.
Light passing through the pupil and the crystalline lens eventually forms an image on the retina. Here much of the light is caught and registered by special neural cells called "photoreceptors." As in human beings, there are two types of photoreceptors in the retina: "rods", which are long and slim, and "cones," which are short, fat and tapered. The rods are specialized to work under dim light conditions. Not surprisingly, dogs have a much higher proportion of rods in their eyes than humans do, but they also have an additional mechanism to meet the needs of night-hunting that is not found in humans.
You might have noticed that at night, when a dog's eyes are caught be car headlights or in a flashlight beam, they seem to glow with an eerie yellow or green. This color comes from the "reflecting tapetum," which is behind the retina and acts as a sort of a mirror. The shiny surface of the tapetum bounces any light that has not been caught by the photosensitive cells back up to the retina, thus giving the photoreceptors a second chance at catching the dim light entering the eye.
The tapetum does more than simply reflect the light, but actually amplifies it through a photoelectric phenomenon called fluorescence. This not only adds to the light's brightness but it also slightly changes the color of the light that is reflected back. The color shift moves the wavelength of the light closer to that to which the rods are most sensitive to and can best detect.
Although the light bouncing off of the tapetum increases the sensitivity of the eye, there is a cost. The light that hits that reflective surface in the back of the eye comes from various directions, and, like a pool ball hitting the bumper edge of the table, it does not return along exactly the same path as it entered, but bounces off at an angle. Because the incoming direction of the light and the reflected direction are different, images on the retina are smeared and appear to be a bit blurred. Thus the dog's eye has clearly chosen to sacrifice its ability to clearly see fine details in order to allow it to function better in dim and dark conditions. Simply put this means that a dog can see more in dimmer light than we humans can, however when there is enough light for the human eye to see something, we see clear images and smaller details.
Stanley Coren is the author of many books including: Born to Bark, The Modern Dog, Why Do Dogs Have Wet Noses? The Pawprints of History, How Dogs Think, How To Speak Dog, Why We Love the Dogs We Do, What Do Dogs Know? The Intelligence of Dogs, Why Does My Dog Act That Way? Understanding Dogs for Dummies, Sleep Thieves, The Left-hander Syndrome
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