Deer pupils are horizontally split, giving them a totally different view of our own. They can take a tremendously wide field of view, with no eyeball movement. Their orbs are designed to pick up danger where it will do them the most good...at a distance.

The slit pupils naturally orients their eyes to pick up movement at or just below the horizon, the place in which a predator is most likely to appear. Their eyes are less adept at picking up movement above the horizon...unless there is obviously a noise for them to look at. Concealment and your motion will either save your ass or give it away.

Thats why I get high as I can in a tree. wait a minute, that sounds wrong. I will go as far up as I can in a tree to be less visible to a deers normal movement and vision.