Field of vision
Dogs' eyes are placed on the sides of the head. Because of this, they have a visual field of 250 degrees compared with the human field of 190 degrees. This means dogs have the advantage of 60 degrees more peripheral vision than humans.
The central or binocular field of vision in dogs is approximately half of that of humans. The binocular field is where the visual field of both eyes intersect. It's where we can focus and judge the distance of an object. In parts of the field where only one eye can see we don't have good focus and can't judge distance, but we can see movement, color, and shapes. Humans accommodate (focus on items at different distances) better than dogs because of the shape of our lenses, and our greater number of color receptors.
Dogs like Labradors, Golden Retrievers, and German Shepherds (breeds commonly used as guide dogs) have a binocular overlap of about 75 degrees compared to about 120 degrees in the human. This is because the humans eyes are set on the front of the head, whereas the dog's eyes are set about 20% apart. Snub nosed dogs have a smaller field of vision.
There are two types of vision receptor cells: rods, which detect light and dark, and cones, which detect color. Humans have more cones but dogs have more rods. Because dogs have no fovea (or area with 100% cones), it is estimated their eye for detail is 6 times poorer than that of a human. On the other hand, because they have so many more rods than humans, dogs can see in light five times dimmer than humans can.
Rods are the vision receptors that detect dark and light. Think of them as receptors that see black and white television. Though they are not able to distinguish color, they are better than cones at distinguising light from dark, and in perceiving light even when it is dim. Dogs have many more rods than humans have, making their night vision superior to ours.
It is a common misconception that dogs are color blind. They are not. They simply see color less strongly than we do (because we have more color receptors) and they only have two kinds of color receptors compared to our three.
Humans posess a macula or central retina area that is 100% cones, whereas the central portion of the dog's retina containes only 20% cones. Therefore, humans are five times more receptive to colors than dogs in the area where the eyes focus best.
Human vision is trichromatic which means our cones come in three colors: 'red', 'green', and 'blue-violet.' Dogs have only two types of cones. Their dichromatic (two color) color vision is similar to that of a human with red-green color-blindness. They distinguish best between yellow and blue, which is why agility obstacles are usually painted in those colors.
A dog's visual acuity is approximately 20:80, compared to our 20:20. That means that what we can see clearly at 80 feet a dog can only see clearly when it is within 20 feet. However, certain breeds, such as Labrador Retrievers (commonly used as guide dogs) are bred for better vision and may have visual acuity close to a human's acuity.
Dogs start with an advantage in having more rods (light receptors) than humans have. They also have a larger pupil than humans have, which lets more light enter the eye.
Dogs posses a tapetum, a mirror-like structure in the back of the eye that reflects light. This structure bounces the light waves back at the retina a second time, increasing the retina's chance to collect the light. It is the tapetum which causes dogs eyes to glow in the dark.
The tapetum, incidentally, is responsible for the blue glow in dogs eyes when they look right at the camera as a flash goes off. Humans produce "red eye" because the flash is reflected off blood vessels in the back of the eye, but dogs produce "blue eye" (or sometimes green or yellow eye) because of the light reflecting off the tapetum. The exact color reflected off the tapetum depends in part on the color of the dog.
Again because of their high number of rods, dogs are superior to humans in detecting motion. They can even see the flicker as the picture reloads on a television. When dogs recognize a person on sight, it is more likely because he recognizes the typical movements of that person than that he actually recognizes a face.