Quote Originally Posted by Claire Senft
If the lens is symetrical then I would expect the diaphram to be in the optical center.
If the lens is symmetrical, the two optical nodes will be symmetrical about the aperture.

In real lenses the rear and front nodal points will never coincide - that can only happen with theoretical lenses of zero thickness. But they can get close...

The reason why there are two nodal points is the necessary thickness of the lenses, wich is difficult to get rid of in anything more complicated than a pinhole. A Fresnel gets pretty close to zero thickness, but with a lot of other "side effects".