What are you looking at?
What’s Your Point of View?
We often get asked, “How well does my pet see?” That is a difficult question to answer but lets give it a try.
To “see” light has to pass into the eye, get to the retina, be converted to an electrical signal, be transmitted to the brain and the brain “process” the signals to produce and image that is perceived. There are five main ways animals with eyes differentiate between objects; brightness, motion, field of view, texture and colour.
Animals have widely different ability to detect light. Cats and dogs have a tapetum in the back of the eye that reflects light to allow their retina to have two goes at detecting a light photon. This is what gives their eyes the green or blue reflection at night and in photos. We don’t have a tapetum so the light just reflects of the blood vessels in the back of the eye, hence we have “red eye” in flash photos. Cats can detect light at a threshold 7 times lower than humans. When light passes through the eye and onto the retina the image is turned upside down and back to front. The tapetum is only on the top half of the eye so it amplifies light from below, i.e. the ground and not the sky. Pets win this one.
Pets 1 – Humans 0
The cells in the back of the eye (retina) that sense light are called rods or cones. Rods are much more sensitive to motion and brightness. Dogs and cats have many more rods than cones and many more rods than we do. As well as helping with low light as mention it also helps them differentiate motion better than us. A study on some police dogs showed that dogs could see a small moving object at 900 metres but could only see that object when stationary at 585 metres or less. Humans have a fovea, a small area of high density of cones right in the centre of the eye that detects motion very well in our direct line of sight, but we are much less sensitive to motion in our peripheral vision. Dogs are much better at detecting flicker that we are. We will watch a movie at 24 frames per second but a dog can detect a difference or flicker at 70-80 frames per second. A reason to upgrade your TV to that 100 hz or even 200 hz TV. I think we would have to call this one a draw.
Pets 1 1/2 – Humans 1/2
Field of View
This is the total area around you that you can see. We have forward facing eyes and about a 200 degree field of view. With dogs and cats it really does vary a bit with breed, but the normally accepted value is 240 degrees. Humans have a greater area in the centre where we have binocular vision, 140 degrees compared to 60 degrees. Cats fall between dogs and humans. Another draw.
Pets 2 – Humans 1
The tapetum that allows animals to have 2 goes as sensing a photon does increase the ability to see in dim light but it does decrease sharpness. Humans (at least when we are young), have the ability to change the focus of our lens. Cats and dogs have less ability to do so. We also have our fovea which is jammed packed with rods allowing us to differentiate between objects very close together. The Snellen Fraction is a common way of describing acuity. 20/40 means when you are 20 feet away from an object you can see it as sharply as a person with normal vision will see it at 40 feet. Dogs are 20/70, that means what they could ‘read’ at 20 feet, you could read at 75 feet. Cats are even worse at 20/100. Humans definitely win this one.
Pets 2 – Humans 2
There is a lot of information about colour vision in animals around and much of it is contradictory. Colour is perceived by the cones. Humans have three types of cones (red, green and blue). Dogs do not have green cones and they are less densely packed than ours. Dogs would not differentiate well between green and red and the longer wavelength colour generally. Cat have the three cones but don’t have anywhere near as many as we do so they will be much less sensitive to colour differences than we are. We win.
Final score – Pets 2 – Humans 3
So as our understanding of our pet’s vision increases the question “What does my pet see?” becomes a little clearer but a lot more complex. Movement and low light they perceive much better than us. The obvious message from all this is your TV isn’t 100hz or greater you need to upgrade, you owe it to your pets.