The Horse’s large, prominent eye are located high in the head, giving a horse an almost 360 degree field of view. Sight is perhaps the most important equine sense, because, as prey animals, horses need to notice the slightest danger instantly.
Horses can only perceive distance when both eyes are focused on an object, and their field of sharpness is long (horizontally) and small (vertically) meaning that they need to ‘bob’ their heads up and down to really take a new object in.
Like all animals, horses have blind spots, areas where they cannot see. One of these is right below their nose, the other directly behind them, hence why you should never approach a horse from behind without using a vocal warning.
So, how does the eye actually work?
Light enters through the pupil, and is focused by a transparent structure called the lens. This is operated by many ciliary muscles, and the light they focus is focused onto a structure at the back of the eye called the retina. This is covered by millions of tiny cells called ‘cones’ and ‘rods’. These two cell types are responsible for different functions, cones allowing colour perception, and rods detecting light.
The greater the number of rods, the better the night vision. Horses have much better night vision than humans, therefore we know they have lots more rods.
Humans have three types of cone in their eye. Red, green and blue. This means they can accurately perceive these wavelengths, giving us the rich colour vision we have. This is called trichromatic vision.
Horses on the other hand, can only perceive two wavelengths, making them dichromatic. Simply missing this one cone makes such a difference to their vision, as shown below.
Primates (monkeys, apes, humans) are the only placental (mammalian) creatures with trichromatic vision. However, birds have even better colour vision than primates! They are tetrachromats, meaning they are capable of seeing ultraviolet (UV) wavelengths! Some creatures are even pentachromats, such pigeons and butterflies!
Horses cannot distinguish red. It is not that they don’t see red, they do, they just cannot discriminate red/green. Please note that all colour tests and simulations in the world could not give you an accurate picture of how horses see, because no-one knows exactly how they see the world.