Horses have both monocular vision and binocular vision, but can’t use both at the same time. In addition, they have blind spots where they can’t see at all. A horse eye is the largest eye of any land mammal.
Its visual abilities are rather unique, and directly related the fact that the horse is a prey animal, and it’s best response to danger is the flight response.
Visual field of the horse
Like most prey animals, the horse’s eyes are set on the sides of its head, allowing it close to a 350° range of monocular vision.Horses are lateral-eyed, meaning their eyes are positioned on the sides of their heads. This means horses have a range of vision of about 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision.
This provides a horse with the best chance to spot predators.
The horse’s wide range of monocular vision has two “blind spots,” or areas where the animal cannot see: in front of the face (making a cone that comes to a point at about 3–4 ft in front of the horse) and right behind its head, which extends over the back and behind the tail when standing with the head facing straight forward. This means when a horse jumps an obstacle, it briefly disappears from sight right before the horse takes off.
The wide range of monocular vision has a trade-off: The placement of the horse’s eyes decreases the possible range of binocular vision to around 65° on a horizontal plane, occurring in a triangular shape primarily in front of the horse’s face. Therefore, the horse has a smaller field of depth perception than a human.
A horse can use binocular vision to focus on distant objects by raising its head when it looks at a distant predator or focuses on an obstacle to jump. To use binocular vision on a closer object near the ground, such as a snake or threat to its feet, the horse drops its nose and looks downward with its neck somewhat arched.
A horse with the head held vertically will have binocular focus on objects near its feet.A horse will raise or lower its head to increase its range of binocular vision. A horse’s visual field is lowered when it is asked to go “on the bit” with the head held perpendicular to the ground. This makes the horse’s binocular vision focus less on distant objects and more on the immediate ground in front of the horse, suitable for arena distances, but less adaptive to a cross-country setting. Riders who ride with their horses “deep”, “behind the vertical”, or in a rollkur frame decrease the range of the horse’s distance vision even more, focusing only a few feet ahead of the front feet. Riders of jumpers take their horses’ use of distance vision into consideration, allowing their horses to raise their heads a few strides before a jump, so the animals are able to assess the jumps and the proper take-off spots.
Visual acuity and sensitivity to motion in horses
The visual acuity of the horse, or how well it is able to see details, is around 20/33. This is slightly worse than the usual 20/20 in humans, but much better than the visual acuity of dogs (20/50), cats (20/75), and rats (20/300). However, it is difficult to test an animal's visual acuity, so the results may vary between studies.
The horse also has a "visual streak", or an area within the retina, linear in shape, with a high concentration of ganglion cells (up to 6100 cells/mm2 in the visual streak compared to the 150 and 200 cells/mm2 in the peripheral area). Horses have better acuity when the objects they are looking at fall in this region. They will tilt or raise their heads, to help place the objects they want to inspect within the area of this visual streak.
The horse is very sensitive to motion, as motion is usually the first alert that a predator is approaching. Such motion is usually first detected in their periphery, where they have poor visual acuity, and horses will usually act defensive and run if something suddenly moves into their peripheral field of vision.
Color vision in horses is only part of the color spectrum
Horses are not color blind, but have two-color, or dichromatic vision. This means they see two of the basic three wavelengths of visible light, compared to the three-color trichromic vision of the average human. In other words, horses naturally see the blue and green colors of the spectrum and the color variations based upon them, but cannot distinguish red.
Research indicates their color vision is somewhat like red-green color blindness in humans, in which certain colors, especially red and related colors, appear more green. The picture above shows how a horse sees a red or a green apple (bottom) compared to how red or green apples are usually seen by most humans (top).
Dichromatic vision is the result of the animal having two types of cones in their eyes: a short-wavelength-sensitive cone (S) that is optimal at 428 nm (pastel bluish-gray), and a middle-to-long wavelength sensitive cone (M/L) which sees optimally at 539 nm, more of a yellowish color. This structure may be because horses are most active at dawn and dusk, a time when the rods of the eye are especially useful.
The horse's limited ability to see color is sometimes taken into consideration when designing obstacles for the horse to jump, since the animal will have a harder time distinguishing between the obstacle and the ground if the two are only a few shades off.
Therefore, most people paint their jump rails a different color from the footing or the surrounding landscape so the horse may better judge the obstacle on the approach.
Studies have shown horses are less likely to have a rail down when the jump is painted with two or more contrasting colors, rather than one single color. It is especially difficult for horses to distinguish between yellows and greens.
Eyeshine from the tapetum lucidum
Horses have more rods than humans, a high proportion of rods to cones (about 20:1), as well as a tapetum lucidum, giving them superior night vision. This also gives them better vision on slightly cloudy days, relative to bright, sunny days.
In a 2009 laboratory study, horses were able to distinguish different shapes in low light, including levels mimicking dark, moonless nights in wooded areas.
When light dropped to nearly dark, horses could not discriminate between different shapes, but remained able to negotiate around the enclosure and testing equipment in conditions where humans in the same enclosure "stumbled into walls, apparatus, pylons, and even the horse itself. "
However, horses are less able to adjust to sudden changes of light than are humans, such as when moving from a bright day into a dark barn. This is a consideration during training, as certain tasks, such as loading into a trailer, may frighten a horse simply because it cannot see adequately.
It is also important in riding, as quickly moving from light to dark or vice versa will temporarily make it difficult for the animal to judge what is in front of it.
Near- and far-sightedness in Horses
Many domestic horses (about a third) tend to have myopia (near-sightedness), with few being far-sighted. Wild horses, however, are usually far-sighted.
How horses change focus
Horses have relatively poor accommodation (change of focus, done by changing the shape of the lens, to sharply see objects near and far), as they have weak ciliary muscles.
However, this does not usually place them at a disadvantage, as accommodation is often used when focusing with high acuity on things up close, and horses rarely need to do so. Instead, the horse often tilts its head slightly to focus on things without the benefit of a high degree of accommodation.