PanoTools:
Re: Re: entrance pupil - nodal point
Dan Slater 2004-Nov-12 07:11:56
Hi Erik;
>
>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
>
>
The virtual image of the aperture stop when viewing from the front of
the lens is the entrance pupil. This is the bright disk of light you see
if you illuminate a lens from the rear. This is normally toward the
front of the lens for most fisheye and wide angle lenses. In these
lenses, it moves further forward, becoming elliptical with increasing
angle from the optical axis. For telephoto lenses, the entrance pupil is
usually toward the rear, and very often outside the physical lens
housing. It may even be behind the camera. The entrance pupil is an
approximation of the center of perspective (what you refer to as the "no
parallax point"). See my other post as to why I am calling this an
approximation.
>
>
>
>>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...
>
>
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
"
The rear nodal point is exactly 1 focal length away from the focal plane
when the lens is focused at infinity. If you turn the lens around
backwards and image an infinite distance object, the front nodal point
will be exactly 1 focal length away from the image. The nodal point
concept is defined for ideal (paraxial) lenses but is only an
approximation for real lenses, particularily lenses with fisheye
distortion, etc.
> <> 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...
>
>
The entrance pupil for a rectilinear wide angle lens generally has
similar pupil aberrations as the fisheye lens although usually to a
lesser extent. For all lenses, the entrance pupil is not a point, but a
surface. For most standard lenses, the entrance pupil is a disk with a
size proportional to f number. The entrance pupil becomes smaller as the
lens is stopped down.
Dan Slater
[Non-text portions of this message have been removed]
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