A horse’s eyeballs are the largest of any land mammal and magnify everything fifty percent larger than we perceive it. http://practicalhorsemanmag.com/article/eyes_091003-11326

A horse’s eyes are located on either side of his head giving them a wide circular view. This is an advantage for a prey animal needing early warning of predators. This panoramic vision is ‘monocular’ which enables them to view their surroundings on both sides, with either eye. http://www.horsewyse.com.au/howhorsessee.html.

The horses’ binocular vision comes into play when both eyes are focused on one object in front. A horse is using binocular vision will usually stand alert, with both of his ears focused on an object in front of him. When a horse sees movement using monocular vision, he will usually turn his head to see with both eyes – switching to binocular vision (to focus on the moving object). When a horse switches from monocular vision to binocular vision, this causes objects to jump and distort – until focused  again. This may cause horses to unexplainably spook. http://www.equusite.com/articles/safety/safetyEyesight.shtml

It takes longer for a horse to adjust to rapidly changing light levels. This may explain why some horses may hesitate to enter a dark trailer or barn from bright sunlight. The sudden changes in light levels may mean their eyes do not have time to adjust. https://www.thespruce.com/understanding-how-your-horse-sees-1887324