wwp@yahoogroups.com:
Re: nodal point question
Markus Altendorff 2005-Mar-29 15:13:00
Larry Coltharp wrote:
> It would seem to me that the nodal point of any camera
> would the the midpoint precisely under the sensor or the
> film plane. As the camera rotates, it's the sensor or
> film plane that is making a 360 spin directly on it's own
> axis. It would appear that if that's the point of final
> capture or focus then that would be the nodal point.
>
> I'm reading about various nodal points for different
> lens. Would someone please explain how this works,
> please. I'm not doubting it but just need an accurate
> explanation.
The "nodal point" (i've gotten really used to that term, but
it's actually the "entrance pupil") is where "the rays
cross" inside the lens. Spinning the lens around that point
does not create parallax in the picture. If you're using a
pinhole camera, then that point would be the pinhole.
However, since lenses are rather complex machines these
days, this does not apply any more. So, some try-and-error
is necessary. Even worse, with fisheye lenses, that point
depends on the angle in the picture - so, if you're shooting
4 around (90 degrees step), then the setting for no parallax
would differ from a setup with 6 around (60 degrees step).
Since the sensor/film plane is "somewhere" behind all the
glass, the location of the sensor itself does not matter at all.
I've recently found a very nice explanation and very good
idea for locating the entrance pupil - use a laser pointer
and aim through the lens.
http://michel.thoby.free.fr/
(some broken links there, but this one shows the idea pretty
well)
http://michel.thoby.free.fr/SIGMA8mm/Alpha%20test%20300D/Nodal%20point%2...
I've used it on a 18-125 sigma zoom and found that the
entrance pupil for that lens is pretty much with the first
"ramp" between the focus ring and the zoom ring. I've
adjusted my panorama head to that (+/- 1 mm), and presto -
instant non-parallax :) - but i've NOT managed to un-distort
that lens' special deformation yet... but that's a different
problem.
Hope this helps?
-Markus