Camera: lens:: set film offset ?

[color=darkblue]b set_film_offset[/b] : Is this function documented somewhere? this is the only stuff I found :

which should be enough provided one understands the associated behaviour… hence the question is:

assuming I have set-up the Film_Size an the FOV for the lens, is set_film_offset merely moving the film (ie the camera image) in the focal plane?

In this case, what does the camera actually see? only part of the scene or a shifted scene??

Does using set_film_offset produce a similar effect as having a dissymetrical frustum (ie slanted frustum)?

[color=darkblue]b setFrustumFromCorners[/b]

Are the corners coordinates supposed to be given in the lens’ coordinates ?
Does the optical axe change (or is still pointing towards Y+)?
In case it changes (?) how then to consider set_near_far (respect to what)?

The point is I’m trying to compare to OpenGL’s approach, the corners should be located in the near plane, ie the plane perpendicular to the axis direction and located at a distance near from the camera optic center. In this case why would LVecBase3f be used instead of LVecBase2f to define ul,ur,ll,lr in setFrustumFromCorners(LVecBase3f const ul, LVecBase3f const ur, LVecBase3f const ll, LVecBase3f const lr, int flags)

Sorry about this bunch of questions, but apparently the doc is a little bit weak on the subject (so do I!)