I've often thought about this. You can notice the shallow DOF effects of your eye when looking at something close up better if you only use one eye. When looking at something far away your eye it seems to reach an 'infinity point' where almost everything past a certain point is in focus. Just like a lens. If that makes sense.
You can spend ages playing with your eye's aperture. Also try "looking" at the OOF bits when using one eye. Your brain can manage it a little before your eye refocuses. Fun
