Posted on May 2nd, 2011 by FindRetrievers.com Admin
Dog vision is pretty different from ours. The human and canine eye are built much the same, but each has modifications that make it suit the individual species’s lifestyle. Humans evolved as a diurnal species (active in daytime) and canines evolved as a nocturnal species (active at dawn and dusk). As a result the human has less ability to see in low light conditions but has better visual acuity, which is the ability to focus so that two objects appear as distinct entities.
A human with 20/20 vision has excellent visual acuity. A typical dog has 20/75 vision which means their vision is 6 times poorer than ours. A dog must stand 20 feet away from an object that a human can see at 75 feet. The dog has to get much closer to the object than a human. This is due to the fact that dogs have fewer cones in their retinas than people. Cones handle color and daytime vision, and rods handle nighttime vision.
These are the two types of specialized receptors in our eyes, rods and cones. Rods are dominant in the retinas of nocturnal animals, and cones are dominant in the retinas of diurnal animals. In animals where fine vision is critical such as humans, a small pure cone area called the fovea is placed directly in the center of your line of vision. The fovea is the part of your eye that you are using to read this text and covers just a small area of your vision. Now try reading with your finger blocking your central vision and notice how difficult it is. This can give you some idea of how it must be like for an animal without a fovea to make out fine detail.
There is also much variation in visual acuity among the different breeds of dogs, as well as among individuals of a breed. You’ve probably seen bulls in bullfights lower their heads before they charge the matador. Border collies do the same thing when they’re herding sheep. They lower their heads below their shoulders and stare at the sheep. The do this because their retinas are different from ours. Domestic animals have a visual streak instead of a human fovea. The visual streak is a straight line across the back of the retina. When you see an animal lower its head to look at something, it’s probably getting the image lined up on its visual streak. Researchers also have found the two fastest animals, the cheetah and the greyhound, to have the most highly developed visual streaks which are dense with photoreceptors, giving them extra-acute vision.
Another huge difference between humans and canines is that canines have panoramic vision. The position of the eyes within the head determines the degree of peripheral vision as well as the amount of the visual field that is seen simultaneously with both eyes. This binocular vision is necessary for judgment of distances. Dogs have eyes which are placed on the sides of the head, resulting in a visual field of 240 degrees compared with the human field of 200 degrees. The central, binocular field of vision in dogs is approximately half that possessed by humans.
The third area where humans and canines differ is the ability to see color and contrast. The perception of color is determined by the presence of cone photoreceptors within the retina. These cone cells function in bright light conditions and comprise approximately 20% of the photoreceptors in the central retina of the dog. In humans, the central retina (macula) is 100% cones. Animals see more intense contrasts of light and dark because their night vision is so much better than ours. Good night vision involves excellent vision for contrasts and relatively poor color vision.
You need to know something about an animals’ color vision to predict what visual stimuli they’ll experience as high-contrast. Birds see four different basic colors (ultraviolet, blue, green and red), people and some primates see three (blue, green, red), and most of the rest of the mammals just see two (blue and green). With dichromatic, or two-color, vision the colors the animal sees best are a yellowish green (the color of a safety vest) and bluish purple. That means yellow is the high-contrast color for almost all animals.
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