Horses have large complex eyes that can see the horizon and the ground at the same time. Because of this, they don’t see clearly at a distance. They see movements and shapes better than specific details.
They are “near-sighted” and their depth perception is not very strong. Rule of Thumb: Eyes on the side of the head = prey; eyes on the front of the head = predators. Having eyes set back from his nose means he can’t see directly in front of himself; he can’t see his nose and mouth. You can probably see your nose but not your mouth.
Horses’ eating habits dictate that their heads be at ground level most of the day. Therefore, they need to see the grass they are eating and scan the horizon for predators at the same time. Their range of vision is about 300 degrees, except for an area of about three feet directly in front of and six feet behind them.
Horses have both monocular and binocular vision. When a horse looks to either side, each eye moves and sees independently. This results in lack of depth perception. When a horse looks straight ahead, both fields of vision overlap and his vision becomes binocular, just like humans, and he gains some depth perception. In order to bring objects into focus, the horse will move his head up or down.
Horses have a complex eye to brain structure. Each eye feeds into one side of the brain with limited crossover, and the pictures the eye receives are not as sharp as are ours. This is an important factor in the training process. This means that while you may cause no reaction when working on one side of a horse, you may cause a reaction on the other.
Horses, with their monocular/binocular vision, may also see objects larger than they really are. You may appear to be ten feet tall to a horse!
Horses have much better night vision than we do. This is possible because of the tapetum, a highly reflective area in the back of the eye. This is what you see when you shine a light into a horse's eyes at night; it is reflected back as a green glow.
Most animals, other than birds and primates, see just two colors (blue and green). The colors these animals see best are yellowish green and bluish purple. Therefore, yellow is a high-contrast color for almost all animals. This may be why animals react strongly to yellow turnouts and machinery.
According to Temple Grandin in "Animals in Translation", horses see the way they do because of the difference in the shapes in our eyes. Human retinas have a "fovea" (foe-vee-a), a round spot in the back of the eye where they get their best vision. Domestic animals -- and fast animals who live on the open plains have a "visual streak" instead of a fovea. The visual streak is a straight line across the back of the retina. Most experts think the streak helps animals scan the horizon.
Many animals see more intense contrasts of light and dark because their night vision is so much better than ours. Good night vision involves excellent vision for contrasts and relatively poor color vision. Animals’ sharper contrast seems to make dark spots appear to be deeper than lighter spots; the reason cattle guards work.
In "An Anthropologist on Mars", Oliver Sacks told about an artist who lost his color vision. It became very difficult for him to drive because tree shadows on the road looked like pits his car could fall into. Without color vision, he saw contrasts between light and dark as contrasts in depth.