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Thread: Tilt, swing and the back nodal point

  1. #1

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    Tilt, swing and the back nodal point

    This is Figure 3-17 from Leslie Stroebel's View Camera Technique, 7th edition.
    Click image for larger version. 

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    If I'm interpreting the figure and the caption correctly, Stroebel is saying that to avoid the need for recomposing and refocusing when using axis tilt or swing, lenses should rotate about a point directly in line with the back nodal point of the lens. Empirically this seems correct because that's how my symmetrical lenses behave on my axis tilt standard. In contrast, my telephoto and reverse-telephoto lenses need recomposition and refocusing on my axis tilt setup.

    The reason for this post is that -- usually in the context of making panoramas with cameras where the image plane and object plane are fixed relative to each other -- it's said that the point around which a lens must rotate to maintain perspective is the entrance pupil. You can find this claim all over the Internet. Here's one example: http://hugin.sourceforge.net/docs/ma...lax_point.html

    To those of you who are expert in the geometry of tilt and swing, I would like to know whether Stroebel is correct, and the other claim is incorrect. Or is it the case that Stroebel is correct for view cameras, where the object plane and image plane are not fixed relative to each other, and the panorama folks using cameras where they are fixed are also correct for their situation.

    Thanks, Rob
    Last edited by rdeloe; 20-Feb-2022 at 14:40. Reason: Spelling

  2. #2
    Nodda Duma's Avatar
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    Re: Tilt, swing and the back nodal point

    They’re both correct.

    From Wikipedia:

    “ The nodal points are widely misunderstood in photography, where it is commonly asserted that the light rays "intersect" at "the nodal point", that the iris diaphragm of the lens is located there, and that this is the correct pivot point for panoramic photography, so as to avoid parallax error.[4][5][6] These claims generally arise from confusion about the optics of camera lenses, as well as confusion between the nodal points and the other cardinal points of the system. (A better choice of the point about which to pivot a camera for panoramic photography can be shown to be the centre of the system's entrance pupil.[4][5][6] On the other hand, swing-lens cameras with fixed film position rotate the lens about the rear nodal point to stabilize the image on the film.[6][7])”


    So if the lens swings and the film plane stays fixed, rotate around the nodal point.

    If the whole camera swings, rotate around the entrance pupil.

    -Jason
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  3. #3

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    Re: Tilt, swing and the back nodal point

    Done and done! Thanks Jason. Crystal clear.

  4. #4

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    Re: Tilt, swing and the back nodal point

    Hello from France !

    Leslie StroebeL is perfectly correct, at least as far as you want to get a sharp image of a far-distant object i.e. an object located several focal lengths ahead. (see below for a discussion on this issue).

    Here we consider how a sharp image moves and de-focuses when the lens is rotated, the object and ground glass being fixed.

    Panoramic stitching is a totally different problem.
    In panoramic stitching the problem is to keep the alignments in object space, when the lens+camera+film (or sensor) is rotated as a whole.
    In this situation we have to deal with projections of un-sharp images on film. Un-sharp images are formed by conical projections on film of the exit pupil, and it can be shown that in order to keep alignments in object space, the proper point of rotation of the whole camera+lens+film is the center of the entrance pupil of the lens.

    So we are speaking about two different problems
    1/ Leslie StroebeL is right (for far-distant objects) regarding the properties of the rear nodal point of the lens regarding sharp images;
    2/ and those who explain that the entrance pupil is the proper point or rotation for panoramic stitching are also right, no contradiction, the two problems are different!

    Regarding the property of the rear nodal (or principal) point of the lens to keep fixed and sharp images of distant object when the lens is rotated, the object and ground glass being fixed, it is not a universal property.

    For example imagine that you take a close-up shot at 1:1 ratio with a perfectly symmetrical lens, then the proper rotation point for the lens, object and images being fixed, in order to keep the image fixed and as sharp as possible, is the center of symmetry of the lens, and not the rear nodal point.

    See attached diagram
    rotation-point-sharp-images.jpg

    The explanation for the role of the entrance pupil in panoramic stitching is bit more complex, it is summarised on the following ray-tracing diagram.
    https://www.flickr.com/photos/43175600@N00/827305231/
    Attached Thumbnails Attached Thumbnails rotation-point-sharp-images.jpg  

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