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Mailinglist:PanoTools
Sender:Erik Krause
Date/Time:2004-Nov-11 22:27:02
Subject:Re: entrance pupil - nodal point

Thread:


PanoTools: Re: entrance pupil - nodal point Erik Krause 2004-Nov-11 22:27:02
On 11 Nov 2004 at 16:35, Rik Littlefield wrote:

> 1. I'm having trouble with the precise definitions here.  I'm getting 
> that the entrance pupil -- the virtual image of the aperture stop -- 
> is an outline enclosing an area, but the center of perspective is a 
> dimensionless point.  Apples and oranges, on the surface.
> 
> Is it more correct to say that the center of perspective is the 
> intersection of the lens axis and the plane containing the entrance 
> pupil?

Yes, for a rectilinear lens this is absolutely true. For a fisheye it 
is not, since the focal length (and hence all the optical geometry) 
depend on the viewing angle. If your fisheye has 8mm focal length, 
this is only true for the center. The focal length gets smaller to 
the edges and is virtually zero at the very edge of the image circle. 
Otherwise it would not cover 180° field of view (or more).
 
> 2. What happens in the case proposed by Erik Krause, where at full 
> aperture the limiting stop is not the variable iris?  If the center 
> of perspective is exactly tied to the image of the aperture stop, 
> then it seems the center of perspective will suddenly shift from one 
> place to another as the lens stops down.  Possible, I guess, but it 
> does not seem likely.  
> 
> What I imagine will happen is that the center of perspective will 
> stay at the same place.  I also imagine that place will be very near 
> the image of the iris, but perhaps *slightly* displaced from it 
> depending on the exact placement of the iris.  (I think it cannot be 
> displaced very much, or the lens would vignette when stopped down.)

This is a misunderstanding. The english term 'entrance pupil' means 
the same as the german 'Hauptebene' (please refer to the links 
previously posted). This is a plane, where the diameter of the 
effective aperture is measured. The location of this virtual entrance 
pupil on the optical axis is not determined by the position of any 
physical aperture (which can be anywhere inside or behind the lens) 
but is a projection of the actual aperture into this plane. 

If the definition of the nodal point John gave is the correct and 
widely accepted one, then the nodal point is the point where the 
optical axis goes through this plane. This point is indeed the only 
one where lightrays that pass through it leave the lens in the 
corresponding point at the exit pupil. This is the point of desire, 
the no parallax point.

best regards
-- 
Erik Krause
Ressources, not only for panorama creation:
http://www.erik-krause.de/



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