I think if our eyes are good there are not many situations where we naturally percieve a shallow DOF. How is it that lenses create this? What happens for shallow DOF to happen in photos and on movies?
Good question I think.
However, I don't agree with "if our eyes are good there are not many situations where we naturally percieve a shallow DOF". I can perceive a blurred background right now, over the top of my monitor, in the distance. If I look at the distance, it comes into focus—just like a camera can change focus. But the difference between a photo and what you see is that with a photo, you can closely examine the out-of-focus areas that the camera captured; with your eyes, you can't really closely examine out-of-focus areas, because your eye will naturally then just bring them into focus.