Sorry for the multiple posts; finding good info and want to share. Take a look at the bottom diagram of this image:
This image shows where stereo images will appear on the screen in order to produce the three depth scenarios. Dark blue represents the subject that pops out in front of the screen. Note that the left image on the projection plane (your screen) goes to the right eye and vice versa. And note how far apart they are. It takes a lot of eye muscles to converge those images (cross-eye, which causes eye strain).
Red/magenta dot is zero parallax, or no depth. This item appears as a single image on the screen and appears right at the screen.
Lighter blue represents depth behind the screen. Distance between stereo images on the screen is much closer together, and more importantly, left image goes to left eye and vice versa. It's also important to point out that the distance between these objects can never exceed the interocular distance (distance between the eyes). If it does, we won't be able to focus on it (can't focus past infinite distance) and there will be terrible eye strain. A lot of bad 3D imagery suffers from this problem when trying to achieve a large depth behind the screen.