Cats and dogs have quite a few differences in their eyes compared to ours. Here are some interesting facts about their vision and ours, and some fun insight into how they view our world.
Can dogs and cats see color?
Human eyes are able to absorb blue, green, and red wavelengths of light, allowing us to visualize the different combinations of these colors. Cats’ eyes have similar light wavelength absorption, although the colors are likely not as rich or vibrant as what we can see. This may be because they have better night vision than we do. Dogs’ eyes can only absorb blue-violet and red wavelengths of light, so their color vision is more limited. Certain birds and fish can actually see wavelengths of light within the ultraviolet spectrum of light in addition to reds, greens, and blues - this means their color vision is richer and more vibrant than even humans!
Night Vision in Cats & Dogs
Cats, and to a lesser degree, dogs, can see better in the dark than humans. Why? For starters, their pupils and corneas (the clear layer that covers the pupil and colored part of the eye, called the iris) are larger than humans’. This allows more light to reach the back of the eye where the retina (the inner layer at the back of the eye) can process it. They also have an increased proportion of cells (“photoreceptors”) that process information in low light, called rods, compared to the human eye.
Vision Precision
Cats’ and dogs’ night vision results in a decrease in their vision sharpness. This is because of the increased proportion of rods compared to human eyes. Because they have more rods, they have the ability to see more in low-light situations, but what they see is not as well focused. Near (or short) sightedness, in which images are focused in front of the retina rather than directly onto the retina photoreceptors, can also contribute to vision sharpness. A study done on 1,500 dogs showed that up to 25% of dogs were near-sighted. Can you imagine seeing one in four dogs at the dog park running around in doggy glasses? Cat vision precision is even worse than dogs. To give you a general idea for vision precision comparison, humans generally have 20/20 vision, which is a measurement called the Snellen fraction. Dogs are estimated to have 20/75 vision and cats are estimated to have 20/150 vision.
Why do cats have slit pupils?
Cats have slit or vertical pupils and dogs and humans have round pupils, but why? We don’t know for sure, but from an evolutionary standpoint, scientists believe the vertical pupil helps with dim or low light vision and may help against glare. Perhaps that is also why cats seem to see a little better in the dark than dogs and humans. Interestingly, big cats like lions, leopards, and cheetahs have round pupils, but foxes, which are in the same genetic family as dogs, have slit pupils.
Cloudy Eyes - Cataracts or Old Age
The lens, a clear circular structure, sits inside the eye just behind the pupil. It allows the eye to focus. Because it is normally clear, it is generally invisible. Humans have lenses too. The lens is the culprit for cataract formation. The normally clear lens becomes opaque, white, or cloudy, distorting or blocking vision. If you look closely, you may notice a diamond or crystal pattern to cataracts. However, in some cases, a cloudy lens is just the result of a normal aging process in which the lens hardens and thickens over time. This is called nuclear sclerosis or lenticular sclerosis. The dog or cat may become a little near-sighted from this aging change, but can otherwise see pretty well. Don’t worry, your veterinarian will be able to tell the difference with a thorough eye exam
Three Eyelids
Dogs and cats actually have three eyelids instead of just the two that humans have. They have an upper and lower eyelid like us, but they also have an inner eyelid, called the nictitating membrane. The third eyelid sits in between the lower eyelid and the eye and usually has a pink or dark brown color. Most scientists believe the third eyelid is used to protect the eye and helps to sweep away any debris that comes in contact with the eye itself.
Glowing Eyes
Why do our pet’s eyes seem to glow at night time? It’s similar to the reason for the “red eye” humans get in pictures from a camera flash. Red eye in humans occurs because a camera flash introduces a large amount of light into the eye, it rebounds inside the eye and gets reflected back to the camera. The red we see in humans is from the reflection of all the blood vessels in the retina at the back of the eye. Dogs and cats (and many other animals) have a group of special reflective, iridescent cells within their retina called the tapetum lucidum. When light hits the inside of their eyes, greens, blues, and yellows can reflect back instead. This likely developed from an evolutionary standpoint to help with their night vision.