Long shots have less 3D simply because of the distance. Your eyes are a set distance apart, so the further away you are from something, the less your stereo vision will matter in relation to that distance. This is why your eyes do not see the 3D of the moon the way they do with a basketball a few feet away, and why we don't see any distinguishable 3D on distant mountains.
If you want to increase the 3D effect of things that are further away, you have to increase the separation between the two cameras (or perspectives, if rendering). This has the effect of making the distant objects look like a miniature, though, because technically it means your "eyes" are further apart. This literally creates an illusion of your head being larger.
It's a fun effect to play with in 3D, though. I do 3D photography and I snapped a shot of Santa Monica from inside an office building. I made the two shots about 20 feet apart, and it gives a great 3D effect which also making the city feel like a tiny tabletop model.
In a movie, it would be a dizzying effect to change the distance between the cameras while shooting a scene. It could be quite psychedelic. It could also mimic the literal result of your own body shrinking or growing.
Also, doing 3D with the cameras closer together can make 3D not normally able to be seen, like shooting insects in close-up macro as though you are also the size of an insect. I have a stunning BBS documentary done this way. You can't do this with your own eyes -- If you hold an ant close enough to your eye to see it close up, it's impossible to see it with both of your eyes in 3D because it would have to be right up to just one of your eyes. You'd also need a magnifying glass.
Parallax aka stereo vision is a very interesting thing, and there is a lot that can be done with it. Movies haven't done a whole lot with it, but there are documentaries with some great experimentation and amazing 3D photography.
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