People used to think that dogs couldn't see any colors, and that in a dog's eyes, the world looked kind of like an old black-and-white movie. But now people know better because a bunch of scientists have been studying the subject of what dogs can see. And what they figured out was that dogs can actually see some colors, but not as many colors as a human can see. In fact, dogs see stuff pretty much the same way that people do who have what's called red-green colorblindness. Here is a picture of the colors people can see and also the colors they think dogs can see. Of course, since I am a dog, both of them look alike to me, but my human readers will probably be able to see the difference.
Anyway, the reason people can see more colors than dogs can is because the two species evolved in different ways so they could find the food and stuff they needed. So humans, who are primates, the same as chimpanzees and monkeys, went around looking for food in the daytime, and they needed to be able to see colors so they could find nice, ripe fruit and other yummy things to eat. Also, they used to live in trees, and they had to be able to see where the next tree limb was so that they could jump there without falling down and breaking their necks.
But dogs were nighttime hunters, so they didn't need to see colors so much. What they needed to see was some animal moving in the tall grass or in the trees, even in the dark. So their eyes evolved to be able to see movement at a distance.
If you took biology class in school, you might have learned about rods and cones, which are these things inside your eyes that help you see. The cones are what you need to see color, and the rods help you see better when there's not much light. People have three kinds of cones, and dogs only have two, so that is why people can see more colors than dogs can. But the 4% of men who are red-green colorblind only have two kinds of cones, just like dogs.
People have this part of their eye called the fovea, which is made of 100% cones, but dogs don't have this. What they do have is a bunch more rods than humans have, so they are lots better at seeing in the dark. And besides that, dogs have something called a tapetum lucidum, which is in the back of the eye, behind the retina, and it reflects light, kind of like a mirror. So when you see a light shine in dogs' eyes, this is why the eyes look all silver and like "alien eyes," which is what Mom always calls them.
Okay, so besides color, there is something called "acuity," which means how well you can see little details. And humans have about six times better acuity than dogs do. In the picture just above, there are some lines, and a human can supposedly make out the separate lines in the top picture, but to me they just look like a gray blur. I can see the bottom lines, though, because they are wider. So anyway, if you compared a human's eyes and a dog's eyes using one of those eye chart things, and the human had 20/20 vision, the dog would have something like 20/75. So this means that dogs are nearsighted.
But guess what! When it comes to seeing movement, dogs are much better than people. Some tests that scientists did with dogs showed that the dogs could recognize their humans moving when they were really far away, like 800-900 yards. But if the humans were standing still, the dogs could only tell them apart when they were 500-600 yards away. Which is still pretty good, if you ask me.
Another thing that helps dogs see movement is that dogs' eyes are more on the sides of their heads than people's eyes are, and that way, dogs can see a wider range of stuff. This wider range is called the "field of vision," and dogs that have long muzzles, such as sighthounds, might have a field of vision that is 270˚. This is much wider than a human's field of vision, which is only 180˚. But dogs with flat faces, like for instance pugs, see in a range that is more like a person's.
Anyway, even though people see better in some ways than dogs do, in other ways dogs are better. And just remember that when it comes to seeing the world through scent, dogs will always be the very best of all!
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