John Richter wrote:
Also, the further the subject is away. The more you have to shift from 1st to 2nd shot. Rule of thumb that I learned was if it's 20 feet and beyond, shift 1 foot. Closer than that means less shift. macro for instance is very little shift. Something a couple hundreds yards away might require maybe 5 feet or somewhere around there, providing your not going for close FG detail. A factor may be a zoom lens as well.
Focal length makes no difference, only parallax defines the strength of 3D. For a realistic 3D view you should never separate the cameras by more than 2.5 inches or so. Any further separation will produce an exaggerated effect, no matter how far away the subject is. However, with distant subjects we don't see very strong 3D with our eyes, because the parallax effect is so small. Consequently, for distant subjects you can 'improve' on human 3D vision by separating the eyes further, but it's not realistic, from a human point of view.