How much can horses see?

Compared to humans, the horse’s eye is huge. In fact, it is the largest eye of any land mammal.

When you are next talking to a friend, have a close look at how the human eyes are placed in the head, and then go and have a really close look at how the horse’s eyes are placed. They’re very different, aren’t they?

If we take a good look at the unique way horses see, it will help us to understand a little of the basic nature of these animals, particularly how, and why, horses react as they do to things in their environment.

Not only are the horse’s eyes huge, they are placed very differently on the head, when compared to humans. Set on the side of the head, they offer stereoscopic vision just as in humans, but they also offer a field of view of almost 360 degrees. Just so that you fully understand the significance of that, stand quietly and look forward into the distance. You will see that your vision offers about 180 degrees, and that you do tend to notice movement right to the periphery of that 180 degree vision.

Now imagine that you could actually see that much again, behind you, as well, so that you took in the entire horizon, even the area entirely to your rear. That’s how extensive the horse’s field of view is. Pretty awesome really.

This panoramic view provided to the horse is, in fact, exactly what you would expect a prey animal to evolve so it can protect itself. A horse holding its head level only has two very small blind spots, which extend a few metres directly behind the rump, and a few centimetres directly in front of the nose.
This almost 360 degree view allows the horse to see almost anything that approaches.

Next time you have a few spare minutes, try to sneak up on your horse. To make it easier for you, try to sneak up in the narrow blind spot directly to the rear of the horse. When you surprise a horse in this blind spot, the horse will be all the more startled because it is very used to not being surprised at all.

It is the placement of the eyes on the side of the head that allow this panoramic vision. The long nose and muzzle allow the horse to continue grazing, yet keep a very careful watch for predators.

This wide vision does have a few problems, though:

a) Because of the long nose, there is the small blind spot on the ground in front of the horse which is blocked by the long nose. Think about the horse you are riding when it is just preparing to raise its forequarters to take off over a jump - there is a period when the horse cannot see the jump. The horse has to rely on a mental image of the jump it got a few strides away, but it actually crosses the jump blind.
How’s that for trust! (You will notice some horses tilt their heads sideways to try to get a good look at a jump just before they cross it, however). Is it any real wonder that some horses baulk at a jump? It’s probably more wonder that so many horses actually jump so many obstacles as well as they do.

b) Having the eyes placed at the side of the head actually limits the amount of the field of view that is seen in stereoscopic, or binocular, vision. (Stereoscopic vision is possible when the field of view of both eyes overlap to show the same scene. This overlap is maximum in animals with eyes mounted to the front, like humans and cats, and is limited in animals with side mounted eyes, like horses and some dogs) Where in humans a large percentage of our eyesight is binocular, the horse sees about 65 degrees in front of it as binocular vision. The rest of the field of vision is seen through one eye alone (uniocular vision).
This limited binocular field therefore limits the depth perception available to horses. This is probably why some horses are confused in front of a jump, because the horse is unable to judge distance accurately. They have no problems judging the width of jumps, because they do this using monocular vision, but managing steroscopic vision, especially when there are many confusing lines leading to a jump, can be extremely confusing for the horse.

c) Humans and other animals that use their hands to manipulate objects have well developed abilities to focus the eyes by using the eye muscles to change the shape of the lens, and hence change focus. The horse has significantly less ability to change focus than humans or dogs, so the horse has less ability to perceive fine detail, except for in a thin strip of the retina. To adapt, you will often see horses moving their heads up and down to see objects in fine detail, by manipulating the image onto the more sensitive area of the retina. When you think about it, that possibly explains why horses suddenly shy at something that has been in its field of vision for some time - the poorly detailed image that has been in view for a while has suddenly moved into sharp view as the horse moves its head and the image falls on the sensitive retinal strip.

d) The large shape of the horse’s eyes and the natural sharp focus on distant objects give the horse a very high level of visual acuity (the ability to distinguish fine details of an object at distance). To put this into perspective, the horse’s acuity is about 20/33, compared to a normal human. That means that detail a human can discern at 33 feet, a horse can discern at about 20 feet. That is better than dogs (20/50) and cats (20/100)

Horses have excellent night vision. The rods (the cells in the retina that are sensitive to dim light) are in higher levels than the cones (the cells of the retina that are sensitive to colour) in the retina. In addition, there is a structure called the Tapetum, a layer of reflective cells in the rear of the retina that act like a mirror to reflect light back into the eye, giving the retina another chance to pick up low light levels entering the eye (this is what makes animal eyes shine in car headlights). The Tapetum is also prominent in dogs, cats, and many nocturnal mammals. Out of interest, the ‘red eye” seen when humans have a flash photo taken is a reflection of the red blood vessels behind the retina, not the tapetum.
While horses can see well in dim light at night, they do not have the detail that they have in daylight, and tend to see movement well without a lot of detail.

Humans, and other animals such as primates, that are able to see in full colour, have three kinds of cones in the retina: each sensitive to a different wavelength of light (blue, green, red). Horse studies show that horses have two kinds of colour sensitive cones, allowing a vision very similar to that in colour blind humans. The reason horses need to see in some colour is simple: the highly sensitive rods that see in dim light can easily be overwhelmed by bright light, while cones require more light to initiate a signal and are better adapted to bright light and daylight conditions. At least rudimentary colour vision allows the eye to see during both day and night conditions. This is of some significant value to an animal used to looking for predators.

Next time your horse shies, or baulks at an unfamiliar image, spare a thought for the unique adaptations that have allowed horses to survive for so long as a prey animal, and think a little about how you would handle being able to see everything around you at once, especially when a lot of it is not in as much detail as you are used to seeing normally.

And then spare a thought for the amount of trust a horse puts in you to keep it safe in unfamiliar environments, often with tremendous variations in light and dark, and in difficult conditions, at speed, when you are simply enjoying a ride. Is it any wonder that some horses shy at high speed traffic on a road, or a kangaroo jumping out of the bush? Is it a surprise that some horses shy when a human handler moves an arm too quickly in front of their face, or approaches a horse quietly from behind?