"You can see what causes that if you look at the houses in the distance and hold your thumb up. Then alternately look through one eye, then the other eye. You see that your thumb shifts relative to the background. It appears in a different place. This eye sees the thumb in front of one house, and the other eye sees the thumb in front of another house, and there's no way I can take those two images and line them up, because they're so different.
"With a large-aperture lens, you can't line up both the near thing and the far thing. If you have the person in the foreground in focus, then the person in the background will be fuzzy. But if you use a very little lens, like a pinhole, then both the near things and the far things can be in focus. To get the image in focus with a large lens, you have to decide whether you want to put the screen where the suns are lined up, or where the branches are lined up.