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  1. #1
    BetterSense's Avatar
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    Do Cirkut cameras rotate around the aperture of the lens?

    I'm building such a moving-film-slot-type panoramic camera out of a 35mm camera. I figure I can then print the negatives with my 4x5 enlarger. I was going to have the camera rotate with the aperture at the center of the rotation. It seemed the obvious way to do the calculations, but looking at currently existing cameras, it doesn't look like they do. It kind of looks like the cirkut cameras, both the aperture and the film plane trace out circles. In my design, the aperture would be stationary and the film would move around it.

    Also, what is a good lens focal length for this type of camera? I only have a 50mm lens for this camera. I think the vertical angle-of-view should be pretty good actually. Are lenses for these cameras of the "long", "normal", or "wide" family?
    f/22 and be there.

  2. #2

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    cirkuts don't move around the aperture. They do usually take into account the position of the nodal point. I don't know the science of it though. With longer lenses the rotation point is pretty far off the aperture point on a cirkut

  3. #3

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    Quote Originally Posted by jamie young View Post
    cirkuts don't move around the aperture. They do usually take into account the position of the nodal point. I don't know the science of it though. With longer lenses the rotation point is pretty far off the aperture point on a cirkut
    Yes, one would think they would rotate around the rear nodal point for the lens in question.
    Bruce Watson
    AchromaticArts.com

  4. #4
    BetterSense's Avatar
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    Yes, one would think they would rotate around the rear nodal point for the lens in question.
    They do usually take into account the position of the nodal point
    What is this "nodal point"?
    f/22 and be there.

  5. #5
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    Not knowing much about a Cirkut except that it's a panning camera, I'm assuming that the lens rotates in space and there's a film transport that runs at the same time to expose a slit continuously... which means the lens would need to rotate about its FRONT nodal point so as to not change perspective throughout the scene. Same principle as used when stitching photos - you rotate about the front nodal point so as not to introduce parallax errors. The rear nodal point is relevant only with respect to the details of the film transport.

    Definition of Nodal Points.

  6. #6

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    BetterSense, the nodal point is where all the light converges at the smallest point... where the rays cross over each other near the center of the lens. The aperture is usually centered there.

  7. #7

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    you have to rotate around the entrance pupil, which is generally not identical to the nodal point:
    http://en.wikipedia.org/wiki/Entrance_pupil

  8. #8
    BetterSense's Avatar
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    Regardless of what it's called, I think I now understand that I should position the camera with respect to the pivot such that there is no perspective change when it rotates. With an SLR, this should be easy to determine by experimentation. It looks like my design will be stuck with a normal 50mm lens.
    f/22 and be there.

  9. #9

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    You can often find the diagrams of lenses online. It's fine if you get pretty close.

  10. #10
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    ...and if you're not close, you get more interesting photos!

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