wwp@yahoogroups.com:
Re: nodal point question
Larry Coltharp 2005-Mar-29 15:53:00
Thanks for all of the help explaining this. Now I understand and can
adjust my setups accordingly.
Larry
>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/>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%20of%20SIGMA%208mm.html>http://michel.thoby.free.fr/SIGMA8mm/Alpha%20test%20300D/Nodal%20point%20of%20SIGMA%208mm.html
>
>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
>
>
>
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