It's easy to think that horses and other animals perceive things exactly like we do. There's no way to know exactly how horses see because we can't see the world with a horse's eyes and brain, but by studying the individual components of the horse's eye, scientists can gain an understanding of what it is capable of. The structure and position of their eyes are somewhat different than humans', which makes a difference in the distance, color, vividness, and visual field a horse experiences.
Many people think that animals, including horses, are colorblind and only see in shades of gray. This is not true. Horses do see color, but they may not see it as vividly as we do. This is because they can only see two of the three visible wavelengths in the light spectrum, which is somewhat similar to the way colorblind humans see. Your horse doesn't see the color red, but it can see blues and greens, so the red apple or the bright orange carrot you offer as a treat may actually appear brownish or greenish to your horse.
If you've ever called your horses in from a pasture in the dark, you'll no doubt have been surprised as they barreled towards you at a wild gallop, but arrived without stumbling over rough ground. They may not see color as well as we do, but they see much better in the dark than we can because their eyeballs have more of the structures that pick up light. If you've taken a picture of a horse with a camera flash, you may see that the horse has ghostly white eyes. This is caused by the tapetum lucidum, a membrane at the back of the eye that reflects light and also aids their night vision. Conditions that would leave us groping for the light switch or flashlight are less worrisome for a horse.
If you've ever walked into the barn at night and flipped the light on suddenly, you'll probably have noticed that the horses blink for quite a long time afterward. This is because it takes longer for them to adjust to rapidly changing light levels. This may also explain why some horses are hesitant to enter dark trailers where they have to go from the bright sunlight to a shadowy trailer. The sudden changes in light levels gives less time for their eyes to adjust.
As prey animals, horses' vision played an essential role in being able to see predators and take flight before they ended up as dinner. Eyes set on the side of their heads–rather than on the front like ours–enable the horse to have almost 360-degree vision. They are unable to see a short distance directly in front of them and directly behind them, which is why one of the safety rules for working with horses is to speak to them when moving behind them. Since a horse has difficulty seeing things directly in front of them, when they are negotiating jumps, a narrow bridge, or other obstacles, they may briefly be doing it while effectively blind. However, because the retinas of their eyes are very large, horses have very good peripheral vision. A subtle turn of the head allows a horse to focus in on an object.
Horses may see into the distance better than we do. It's also likely that they can see motion with greater sensitivity than we can, which is very important for spotting predators before they become a real threat. Horses also seem to be able to see things in more detail than many other animals.
Horses may have brown or blue eyes, with brown eyes being far more common. Appaloosas, Paints, Pintos, and other horses with lots of white on their faces will sometimes have blue eyes. There's no difference in the vision of these horses, although some people may feel they look more likely to spook.
Natural Eye Protection
The protective layer in the corner of a horse’s eye, called the nictitating membrane, helps prevent irritation from dust and objects like grass seeds and stems. You'll often see a bit of tearing and grime in this area that can be wiped away with a soft damp sponge or cloth as part of your grooming routine.
Thinking that horses see the same way we do would be a mistake. It's important to understand how they perceive the world, why they react the way they do to shadows and changes in light, and the extent of their close-up and distance vision. When we're doing things like designing stables, loading horses on trailers, or riding out on trail, it helps to understand what those things look like from the horse's point of view.