PanoTools:
Re: entrance pupil - nodal point
Erik Krause 2004-Nov-12 18:38:06
On 11 Nov 2004 at 22:11, Dan Slater wrote:
> The nodal points are unrelated to the entrance pupil. A good definition
> was posted by John Houghton and is quoted here""
>
> My Higher Physics text book says: "In any thick lens or system of
> lenses there are two points, called nodal points, such that a ray
> incident toward one nodal point will emerge as from the other nodal
> point in a direction parallel to the original direction".
>
> In diagramatic terms:
> http://homepage.ntlworld.com/j.houghton/nodal_pt.gif
Ok, many things are much clearer now, many thanks. There is one thing
I still don't understand. Given the above definition is true (it
should be since it is exactly what I know from school ;-). Given an
object A and B located exactly one after the other on the optical
axis. The image of both will be on the optical axis, too. Now I
rotate around a point on the optical axis, but different from the
above defined nodal point.
A light ray from A will go through the nodal point and hit the film
in point A'. If the point of rotation and the nodal point are
different, a light ray from B that goes through the nodal point will
have a slightly different angle and emerge from the rear nodal point
with that same angle (according to the above definition) and therefor
create a point B' on the film that is not identical with A'.
For my limited knowledge of geometry this means that the nodal point
and the rotational point have to be the same to avoid parallaxe.
I think it is legitimate to look at the central rays only, since
parallaxe is a problem inside depth of field only. The blurred speck
of a point detail is a projection of the effective aperture shape and
might be oval or cut off at one side due to vignetting. This might
look as if it shifts slightly in position, if rotated from one side
of the image field to the other because the cut off changes sides.
This is exactly what the definition of 'entrance pupil' on
http://www.mellesgriot.com/products/machinevision/lef_4.htm
says. In my opinion the definition as "virtual image of the aperture"
is a bit vague and misleading since it not only implies a viewer that
looks from infinity (or a collimator) and depends on the exact
location of the physical aperture.
However, the thick lens model mentioned there and the above
definition of nodal point are valid only for rectilinear lenses
without distortion. A fishey projects 180° field of view in object
space to a far lower angle in image space. Otherwise it would not be
possible to map 180° on flat film.
best regads
--
Erik Krause
Ressources, not only for panorama creation:
http://www.erik-krause.de/
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