PanoTools:
Re: entrance pupil - nodal point
Rik Littlefield 2004-Nov-12 09:42:27
OK, so quickly summarizing what I think I hear...
The "no-parallax point" is a useful approximation that depends on
angle of view for some lenses.
This point coincides with the center of the "entrance pupil", which
is the virtual image of the aperture stop (iris), under the very
reasonable assumption that the lens designer positioned the iris at a
place where it will not cause vignetting. This point can lie pretty
much anywhere, for example from almost the front of the lens (for
fisheye at 90 degrees off-axis) to infinity behind the camera (for
some telecentrics).
The no-parallax point can be found by the classic method of lining up
foreground/background detail. It can also be found by pointing a
laser beam into the lens and looking for maximum transmission, as
described at http://michel.thoby.free.fr/Liste-experiments1.html .
The "front nodal point" is quite possibly someplace else, and for
shooting panoramas we really don't care where.
Putting other aperture stops in the system causes the situation to
become confusing, and we probably don't care about that either.
Does that about cover the major issues?
--Rik
PS. This is not intended to be tongue-in-cheek. I have learned a
great deal from this discussion, and I am indebted to Dan and Erik
and John and Milko and everyone else, including Manfred, who probably
did not at all intend to provoke such an extended discussion!
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