 THROUGH A HORSES EYE
By Walter Berger © 2001
This could also be called “Now you see it, now you don’t”, because that’s how a horse sees. Intrinsically a horse can see just about everything around it.
 It has an approximately 3 degree blind spot behind itself, and about an 65 degree area in front that is covered by both eyes (binocular vision). This leaves an area of 146 degrees on either side to be covered by the left and right eyes. Oh, and there is the bit right in front of him where its nose gets in the way.
This means that you should not sneak up directly behind a horse. Also a horse can’t see what it is eating or what is about to step on. He works from memory and touch.
That can get tricky when you are about to jump something. A horse has limited stereoscopic vision, 10 centimeters depth from 2 meters. He also gets a lot of his visual clues from interpretation and perspective of what he sees. Therefore the need for ground lines when jumping. Look at the diagram and the two horizontal lines. Which appears bigger?
Horses can be fooled in the same way. Other factors that affect a horse's ability to see apart from depth perception is the ability to focus and the amount of detail a horse can perceive. A horse’s eyes are naturally focused for distant objects. He has limited ability to refocus his eyes on close up objects. Also most of the nerve endings and light receptors of the horse are located within a narrow strip along the retina of the eye. In fact the concentration of the receptors there is up to 100 times greater than in other areas of the eye. An image falling outside of that area is not seen as bright or in as much detail.
This means that if a horse is eating grass or doing dressage nicely rounded and concentrating on you (wouldn’t that be nice) he can’t see his surroundings all that well. How often have you ridden a horse when suddenly he shies at a rock, bit of wood or fence post that you have seen for the last 100 meters. It could be because he has moved his head in a way that allowed the image of the dreaded object to fall on the sensitive area of the eye. He then lifts his head and turns it towards the object to utilize his stereoscopic vision to evaluate the monster.
Then again he has probably seen the monster a thousand times, and is just a bit energetic.
Source: The Nature of Horses, Stephen Budianski TOP
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