Why do dogs sometimes keep barking when they see someone they know approaching and don't recognize him until they are very close, are they nearsighted, and what is the world like in their eyes? Some people say that the world of dogs is black and white, and some say that dogs are color blind, they see things in a different color than we do. Yet what is the truth? The overall structure of a dog's eyes is similar to a human's, but there are still some differences. Here's a guide for owners to find out how the world is seen by the dogs around them.

I. Dogs are all nearsighted

Christopher Pirie, a professor and pet ophthalmologist from Tufts University, suggests that dogs are all nearsighted compared to humans. This is because the ciliary muscles in dogs' eyes are less adjustable and cannot freely adjust their focus. They can often only see objects at a certain distance. So dogs perceive the outside world generally rely on the sense of smell, and the visual dependence will be lower.

If human vision is 20/20, then the dog's vision is 20/75, meaning that we can see objects at a distance of 75 feet, while the dog has to walk 20 feet away to see the object. If the dog measures our vision table, it is estimated that only the top two rows can be seen. The degree of myopia varies from dog to dog, with German Shepherds and Chenrezig having a deeper degree of myopia. So sometimes dogs don't react when they see their masters but actually can't see them until they smell them.

II. Is the dog color blind?

Dogs and humans both have retinal cells in their eyes, which are used to capture light as well as recognize colors, and dogs have fewer retinal cells in their eyes than people, so dogs can see fewer colors than people. Studies have shown that dogs' visual cone cells are more sensitive to dark blue light, and people can see red, yellow, and blue, while dogs cannot see red and green.
However, even if a dog cannot recognize some colors, it can recognize objects by controlling the brightness of light because dogs have more optic rod cells in their eyes, which are photoreceptor cells like the cone cells that can sense the intensity of light and help dogs see clearly at night. For example, a red light will have a different brightness than a green light in a dog's eyes. Also, dogs are better at distinguishing color than we are. Sometimes what looks like one yellow to us may look like ten different yellows to a dog.

Three, dogs have a more open field of vision

Generally speaking, dogs have a much wider field of vision. When they run forward, they can see nearly 240 degrees in front of them, while people can only see 180 degrees, meaning that dogs can see objects to the left and right without turning their eyes. The reason for this is that the dog's retina has a high density of optic cone cells, also known as visual striae, which helps the dog to see all corners.
And while most dogs with short noses like pugs don't have a visual band, they do gather these high-density optic cone cells somewhere in the retina, so these dogs can even see the facial expressions of people on TV.

IV. Dogs are more sensitive to moving objects

Although dogs are both colorblind and nearsighted, they were still a good hunting partner for the ancients because of their sensitivity to moving objects, which allowed them to run and chase prey at the same time. Most dogs can see their master's hand signals from a kilometer away, yet they tend to ignore the stationary objects around them. When the owner plays ball-tossing with the dog, the dog can accurately identify the location of the ball, so ball-tossing is also one of the exercises to train the dog's eyesight.
This feature makes dogs hunting dogs, police dogs, and watchdogs, they can quickly catch prey, criminals, and home thieves, any subtle movement may be captured by the dog. But sometimes it's not all good for dogs, because dogs chase cats and other small animals, and even cars, which is why dogs are prone to car accidents.
The dog's world is not as colorful as ours, but that means they can see another world, and even if the dog is nearsighted plus color blind, they can live by smell and hearing.

V. A dog's vision is very different from a human's vision

Dogs see the world in fewer colors than we do, but that doesn't mean that our canine companions are completely colorblind. But even if a dog's visual world is not as clear or colorful as ours, their ability to see movement is superior.

What colors can dogs see?
The human eye works thanks to three types of color-detecting cells called optic cones. By comparing the way each retinal cell is stimulated by incident visible light, our brains distinguish red wavelengths from green wavelengths and blue wavelengths from yellow wavelengths. Like the eyes of most other mammals, dogs' eyes have only two types of retinal cells. These allow their brains to distinguish between blue and yellow, but not red and green.

Dogs are not completely color-blind, but their eye structure is similar to that of red-green blind humans, whose eyes also lack the third type of optic cone cells normally found in humans.

If we assume that their brains interpret signals from the optic cone cells like the brains of colorblind people, we can understand what dogs see.


To see blue and yellow, dogs and humans rely on neurons inside the eye called the retina. These neurons respond to yellow light detected in the retinal cone cells (also located within the retina), but when blue light hits the retinal cone cells, the neuron activity is inhibited. The dog's brain interprets the excitation or inhibition of these neurons as the sensation of yellow or blue, respectively. However, in dogs and color-blind people, both red and green light have a neutral effect on neurons. Since there is no signal to interpret these colors, the dog's brain does not perceive any colors. Where you see red or green, they see shades of gray.

"Humans lose the sensation of red and green," says Netz. "But whether dogs' senses are missing red and green, or whether their brains assign different colors, is unclear."

In addition, like color-blind people, dogs may use other cues to distinguish the color we call "red" from the color we call "green.

Many times, there are good cues to help them figure it out; for example, red objects tend to be darker than green objects," says Netz. "So if it's a dark apple, a red-green blind person will know it's probably red, and if it's a light-colored apple, it's probably Granny Smith."

There is some evidence that dogs may be able to see colors that are invisible to humans. a 2014 study published in the journal Proceedings of the Royal Society B found that the lenses in dogs' eyes transmit large amounts of ultraviolet light and that these wavelengths are blocked by human lenses. This suggests that dogs may see more blue light than we do.

How sharp is a dog's vision?
In addition to lacking some of the hues perceived by the human eye, dog vision lacks some of the sharpness of human vision. In a 2017 study published in the journal PLOS One and conducted at Link枚ping University in Sweden, researchers devised a canine vision test similar to the one ophthalmologists perform on people. Instead of having to recognize letters of reduced size, the dogs were rewarded for correctly identifying images containing vertical or horizontal lines with decreasing amounts of space between them.

The researchers found that the dogs - or at least the Wheaten, pug, and a Shetland Sheepdog that participated in the experiment - were very nearsighted. The experiments showed that the dogs' vision was about 20/50 in well-lit conditions. this means they had to be 20 feet (6 m) away from something to see it, as well as 50 feet (15 m) away from the same object in person.