Dogs see only about one-sixth as well as humans.
Humans like to think that we see well -- a vanity we cultivate by comparing ourselves to dogs, rats, and other mammals that mostly see in black and white, grays, light yellows, and blues.
In fact, most mammals have a sharply reduced visual spectrum because early mammals went through a very long period when almost all of them lived an entirely nocturnal existence.
Most mammals are still nocturnal.
If you wonder where the raccoons, possum, deer and fox are in your area, the answer is that most move about only at night. The rest of the time they are resting up in thick tangles of growth, tucked up into underground dens or hollow trees, slumped into small depressions and wallows, and otherwise "loafing on the couch," just out of eyesight.
Since most humans never stray far from car or path -- and almost never venture into dense tangles or on to even slightly swampy ground -- the critters remain undiscovered.
Animals that are mostly nocturnal do well with eyes that see very little color.
The canine eye is a good example. Though your dog may sleep all night (and half the day too!) it is still only a few thousand years removed from its wolf or proto-wolf progenitor; not enough time for physical evolution of the canine eye to have occurred.
Mice and rats mark their burrows and runways with urine scent trails because, like most mammals, they do not have very good eyesight.
The bottom line is that dogs do not see very well -- not even sight hounds or seeing eye dogs. In fact, dogs seem to see only about one-sixth as well as humans.
Not all mammals see in a reduced spectrum, however. The progenitors of old-world apes experienced a mutation millions of years ago that resulted in more cones and rods being added to their eyes. The result is a wide range of color vision among old-world apes (chimps, gorillas, orangutans) and their cousins, we humans.
Surprisingly, some of the oldest and most primitive animals on the planet have a wider spectral range than humans and chimps. This includes reptiles and the likely descendants of dinosaurs -- birds.
Anyone who has tried to stalk a wild turkey knows how incredibly sharp their eyes are. In fact, all birds have remarkable eyesight. It's been estimated that a common sparrow hawk, for example, could read a newspaper from 25 yards away, while an eagle can spot the twitching ears of a rabbit from a distance of two miles.
The reason avian acuity is so high is that a bird's eye is built entirely differently from that of a human or a dog. For one thing, the retina of a bird is enormous.
The retina is the screen at the back of the eye on which an image, coming through the lens, is focused. This screen is composed of light-sensitive rods and cones, with the rods registering shapes and the cones differentiating colors.
On rods alone, the avian eye far eclipses that of the dog or the human. A human eye, for example, has about 200,000 rods, while an eagle has about a million -- five times more.
The true knockout blow for birds, however, is in the cone department. It turns out that many birds, and especially birds of prey, have cones in their eyes that enable them to see a much wider spectrum than we can, and for some birds -- such as the American Kestrel or sparrow hawk (actually a kind of small falcon) -- this spectrum includes ultraviolet light.
What's the benefit of being able to see in the ultraviolet spectrum?
Simple: mouse urine glows purple in ultraviolet light (one reason "black light" is used to find old pet urine stains on wooden floors).
When a sparrow hawk hunts along a hedge, it is able to look for urine stains that mark mouse holes, runways and nests.
Mice and rats mark their burrows and runways with urine scent trails because, like most mammals, they do not have very good eyesight. Scent cues help rats and mice orient themselves in the complex world of hedge, forest, field and barn. The result is that a rat or mouse can "run" a scent trail very fast without spending time in the open trying to figure things out.
Experienced human ratters know this about rodents, and so they will drag their boot across the middle of a shed and spin long boards perpendicular to where they have been lying, all in an attempt to break up or disturb invisible scent trails. A few seconds hesitation by a confused and disoriented rat is just the edge a terrier needs to even the odds on the farm.
For a sparrow hawk, visible urine trails enable it to focus on areas where small rodent activity is heaviest. By simply flying over a hedge or field, and then lighting on a fence post, telephone pole, or dead tree near visible urine marks, a sparrow hawk can dramatically increase its chance of finding lunch.
Once activity in the grass is spotted, a sparrow hawk will face into the wind and hover low over a grassy area -- a tactic that earned it the name "windhover" or "wind fucker" in 17th Century Holland, when the terminology was considered a little less offensive that it is today.
As for ultraviolet vision, it's not all about food -- it's about sex too. It seems that all those rather drab-looking birds we see in the hedge look a lot prettier when they are seen through the lens of an ultraviolet-sensitive eye.
Birds species in which male and females look very much alike to us look very different to the birds themselves, as they have twice as many cones in their eyes and so can see shade and colors we miss altogether.