There are two types of cells in the retina that receive light: rods and cones. Rods are for sensing motion and work best in low light conditions. All mammals, including people, have more rods than cones. Cone cells are adapted for vision in brighter light and can detect different colors. Humans have three types of cones. Dogs have two types of cones. Evidence suggests that the dog has vision similar to a human who is red-green colorblind. Cats have three types of cones, like people, but do not have exactly the same color vision as we do.
Dogs and cats appear to respond to blue and yellow best, and seem to have more trouble with green and red. What appears red to us is simply dark to the dog and cat, and green light is almost indistinguishable from white (a shade of gray). Colors that would appear very rich to us are probably more pastel-like to the cat. The cat would see a green, grassy lawn as a whitish lawn, and a green rosebush as a whitish bush with dark flowers.
Visual acuity is the ability to see the details of an object separately and unblurred. Acuity is measured in “cycles per degree”, which means how many lines you can distinguish as being separate in a degree of the visual field. Humans see 30 cycles per degree, horses 18, dogs 12 and cats 6. Acuity in dogs is 0.4 times that of people, 0.67 times that of horses, and twice that of cats. Acuity in cats is 0.2 times that of people, 0.33 times that of horses, and 0.5 times that of dogs. If normal human vision is 20/20, then that of the dog between 20/50 to 20/100, the horse 20/33, and that of the cat is 20/100. However, it is difficult to measure acuity in animals so studies have often shown wide variations in results.
The picture demonstrates the idea of acuity and how it differs between humans (top) and dogs (bottom). Humans can distinguish a lot more “cycles” in the same picture. To a dog, the upper image would be a gray blur with no alternating lines.
SENSITIVITY TO LIGHT
The canine and feline visual systems are adapted for performance under low light conditions. These animals have large corneas and pupils to collect more light in dim light conditions. They also have a reflective structure at the back of the eye called the tapetum which reflects light back out of the eye. This way, the retina gets two chances to capture each photon of light. A cat’s tapetum reflects 130 times more light than the human eye. This is why we see the shiny dog and cat eyes in photographs and at night when headlights or other types of light enter the eyes. Cats can detect light that is 6 times dimmer than that which normal humans can detect. Dogs also detect much lower levels of light than humans (but not as low as cats).
Dogs and cats are also very sensitive to motion, especially when compared to an object that is not moving. Some dogs were shown to recognize a moving object at 800-900 meters. If the same object was stationary they only recognized it at 500 meters. People are also more sensitive to motion than to objects that are standing still.
FIELDS OF VIEW
Dogs and cats can see about 150° around from their nose. The horse can see about 350° around because their eyes are positioned to the side of the head. This makes horses better able to watch for potential predators. Humans have a smaller field because our eyes are directed straight ahead. Binocular vision (stereo vision) occurs where the visual field of the two eyes overlaps. In the binocular field depth perception is improved and vision is better. Dogs and cats have some binocular vision but not as much as people. In dogs and cats the binocular field is 85°, in horses it is around 65°, and in people it is around 120°.