This Teapot example shows what can go wrong.

The separation between the two points of view affects the way you perceive the final image.
Too small a gap between the points of view and there is a reduced stereo effect.
Too large a gap and your brain starts to produce an image that looks smaller than it ought to be. Resulting in big objects that look like toys.

In this example I used two 35 mm cameras mounted landscape-wise, side by side on a bar. A standard setup and quite easy to achieve, with two cameras.
However, I was quite close to the teapot as I wanted it to fill the frame. The bar had a slot which allowed one camera to slide along. It was possible to place one camera body behind the other so that the separation between the lenses was reduced. This would, I thought, in turn reduce the toy like look to the teapot.

Yes. It did reduce the toy effect, but it introduced another more destructive element. This is something that is present all the time when you focus a single camera or move a stereo camera.
The fault I introduced was a difference in the size of the object in each image. The camera at the back produced a smaller image than the camera at the front. Simple when you think about it.

If you are near to something it occupies more of the field of view. So from a cameras point of view the object is bigger in the view finder. It occupies more of the film emulsion. A similar effect occurs when you focus a camera with a view finder that looks through the lens. You can see the change in image size as you change focus from near to far, though the image may not be sharp. The lens is being moved relative to the film ... so an image occupies more or less of the emulsion.

Copyright © Dave Dillon 2002