PanoTools:
Re: entrance pupil - nodal point
John Houghton 2004-Nov-12 08:34:13
--- In #removed#, "Erik Krause" <#removed#>
wrote:
> 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.
Erik, If I understand correctly what you are saying, then I think
this is incorrect. It is not generally true that the nodal point is
the point of desire. This animated diagram will show what happens
when you rotate a thick lens with an aperture in front (in this case,
the entrance pupil) around the front nodal point, which is inside the
lens.
http://homepage.ntlworld.com/j.houghton/nodalmov.gif
The entrance pupil moves from side to side and sees around the near
object, which appears to move relative to the far object. I.e. it
exhibits parallax. Rotating about the entrance pupil eliminates this
parallax error. However, it is evident that this example is
unrealistic for fisheye lenses, since simply putting a small aperture
in front of the lens doesn't enable you to get anything remotely like
a full 180 degree image. I agree that the best rotational point is
then going to be a compromise, and needs to be chosen such that
minimum parallax errors occur at the angle of yaw where the seams are
to be placed.
John
John
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