PanoTools:
Re: entrance pupil - nodal point
Erik Krause 2004-Nov-11 11:29:54
On 10 Nov 2004 at 22:16, Dan Slater wrote:
> Hopefully this will help clarify the entrance pupil / nodal point issues
> as related to stitched panoramic imaging:
>
> 1. The center of perspective is the lens entrance pupil,
Ok, I found the reason for my misunderstanding. The german
translation for 'entrance pupil' is 'Vordere Hauptebene' as anyone
can see from a comparison of this very good description:
http://www.mellesgriot.com/products/machinevision/lef_4.htm
and
http://foto-net.de/net/objektive/linsen.html
(see Zeiss Distagon diagram)
> which is the
> virtual image of the aperture stop when viewed from the front side of
> the lens.
I agree, although the term 'virtual image' is used in a bit different
way usually. It describes the image we see if we look through a lens.
This image is smaller than the object we look on, if the lens is of
concave type and bigger if it is of convex type (magnifying glass).
This 'virtual image' is certainly not the 'no parallaxe point',
because it is located behind the actual aperture (nearer to the the
film/sensor), if we look into a lens with convex type front element
(usually a telephoto lens, where the entrance pupil is in front of
the front lens).
See here f.e.: http://www.phys.ufl.edu/~selman/Demo5.html
> 2. The entrance pupil location is generally not at the nodal point
> location. For most stitched image applications, the nodal point
> locations are not important.
What is the exact definition of nodal point? I assumed it identical
to the entrance pupil...
> 3. The entrance pupil is not a point but a complex shape for wide angle
> and fisheye lenses. Entrance pupil aberration (the shape of the entrance
> pupil surface) causes the perspective center to shift forward with
> increasing angle from the optical axis in fisheye and wide angle lenses.
I Agree. Although it should be a point for a rectilinear wide angle
lens, since this type of lens should not change the focal length with
the viewing angle...
> If you stop down a fisheye lens, look at it from the front and
> illuminate the rear with a flashlight, you will see the entrance pupil
> as a bright dot. As you look at it more from the side, you will see it
> move forward toward the front surface of the glass. This is the reason
> that you should measure the entrance pupil position at the angle where
> you will be forming the image stitches. Equivalently, the lens viewpoint
> is from a more forward point when looking sideways (edge of image) than
> when looking straight ahead (center of image). This perspective center
> shift is on the order of an inch or so for typical fisheye lenses.
Fully agreed.
> 4. The perspective center (entrance pupil ) may be inside or outside the
> lens. A lens designer can position the entrance and exit pupil locations
> where needed for a specific application. In fisheye and wide angle
> lenses, the perspective center (entrance pupil) is generally toward the
> front of the lens.
Fully agreed.
> 5. A telecentric lens has either or both the entrance pupil and exit
> pupil at an infinite distance from the lens. In the case of object space
> telecentricity (entrance pupil at infinity), the center of perspective
> is at an infinite distance from the lens, meaning that the lens provides
> an orthographic (perspective free) view. This type of lens is often used
> in machine vision systems to maintain a constant magnification of the
> object for any depth.
Fully agreed.
> 6. Lenses that are telecentric in the image space do not change the
> field of view as they are focused at different distances. For panoramic
> cameras, this would mean that the working focal length would be
> completely independent of the focus distance. Another application of
> image space telecentricity is to eliminate color fringing in 3 chip
> color CCD cameras.
Fully agreed.
Many thanks for that clarification.
best regards
--
Erik Krause
Ressources, not only for panorama creation:
http://www.erik-krause.de/
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