Of all land mammals, horses have the largest eyes. Of all animals, their eyes are fourth largest – just behind ostriches, seals, and whales. With eyes located at either side of their heads and only a very small blind area, horses have a wide, circular view of the world which enables them to detect stalking predators sneaking up from behind them.
This panoramic view actually incorporates two types of vision – monocular, and binocular. The horse’s vision is mostly monocular, with each eye providing a view of what is happening on the corresponding side, and a small binocular vision area that provides a view of what is happening right in front of the animal’s face. Interestingly, binocular vision is directed down the nose toward the ground, rather than straight ahead, and there is a small blind spot that’s about the same width as the horse right in front of the animal’s forehead.
Horses’ large eyes provide them with a great advantage, as they enable the animals to detect even the slightest motion, which is why they can be uneasy on very windy days as well as when they are at a racing event. Horses move their heads up and down to get a better view of things that cause them to feel concern, as they enjoy better depth perception when the head is held at a certain angle. In general, they see best when binocular vision is employed. To get a feel for how horses see, close one eye and walk rapidly toward a wall. You’ll notice that it is hard to tell how far you are from the wall. When you open your eye, you’ll be instantly rewarded with better depth perception, courtesy of your own binocular vision.
At one time, horses were believed to have poor vision, but this is only because early researchers didn’t understand how horses see. In addition, they were originally believed to be short sighted, but this too has been proven wrong. If you’re watching a horse as he sees something moving, he’ll lift his head and gaze into the distance, employing his long range vision, which is even better than our own. Researchers using the Snellen scale to compare horse vision with human vision have determined that horses have 20/30 vision, while a human with perfect vision has 20/20 vision, a dog has 20/50 vision, and a cat has 20/75 vision.
Horses eyes cannot adjust to darkness quickly, which is why they may panic when taken from a brightly lit paddock to a float or a building that is dark inside – they are essentially blinded for a short time while their eyes adjust.
Understanding how horses see helps thoroughbred race horse trainers to keep these equine athletes focused by the use of winkers, blinkers, and pacifiers during training and on race days.