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The Nodal Point - part 1

Posted 01-05-2008 at 01:26 PM by Frank Lopes
Updated 01-06-2008 at 05:07 PM by Frank Lopes

When I started shooting panos, I noticed that some of my assembled images did not quite align properly.

It was not the camera, the software, or, I thought, the technique used. In all cases I was shooting using the same camera and the software was the same.

Assuming it was the software that was doing a lousy job, I experimented with all packages that were available. At one time I had six different trial packages installed...

The problem "stitches" all seemed to have one thing in common: the photos captured both very near and very far objects. Somehow the near objects never seemed to align properly. I would end up with gaps or the near objects (or part of them...) would appear twice.

This misalignment however never appeared when I was shooting scenic panoramas where the subjects were all very far away.

I couldn't understand what it was until someone asked me if I was pivoting the camera on the Nodal Point of the lens.

Since I had no idea what the "nodal point" was, I had no idea what the answer was...

Google here I come!

After some research I found the issue had to do with the physics of optics and parallax.

Light coming into a camera gets reversed. Just like a human eye, what it up is captured upside down, what is left is captured on the right and so on. This switch takes place in the pupil of the eye and, in the case of the camera, inside the lens being used. Where exactly is this point, will be covered in a later blog entry.

So, why does this happen?

Click the images to see them in larger size.

Image 1 - Shooting straight ahead.



First some landmarks:
Camera is on a tripod shooting subjects 1 and 2. The red dot represents the center of camera body, which is the typical pivoting point since it is this point that attaches the camera to the tripod head. The film/image sensor is represented by the heavy blue line and the light rays coming into the camera, in yellow. The two light blue lines represent the X and Y axis of motion of the camera crossing at the optical center of the lens: the nodal point. Inset, is an example of what a photo would capture with the camera in that position.
Notice that the two subjects are very far apart but when viewed from the camera's point of view, their outlines almost touch, but not quite.
Assume this was the first of two shots that will be assembled later.


Image 2 - Pivoting on the nodal point.



Assuming you are pivoting the camera on the nodal point of the lens ( the green dot ), the second shot now shows what happens if you rotated it 15 or 30 degrees to the right.
The subjects now appear on the left side of the photo obviously, but the positions of the two items relative to each other are the same as in photo 1. Notice how the red line demonstrates this in comparison to image 1.
When it comes time to assemble all the two photos, there will be no misalignment issues.


Image 3 - Pivoting on the camera center.



Assuming you rotate the camera on the point of contact with the tripod head ( the red dot ) and shoot a photo, you will now be moving the nodal point "away" from the subjects. Notice how the red line now goes over the closest subject.
The photo clearly shows now subject 2 covers most of subject 1 due to parallax errors introduced when the nodal point was moved away.
Regardless of how good your software is, image 2 will never properly align with image 1.


The moral of the story

If you shoot panoramas and you are capturing both very close and very far subjects, you must rotate the camera on the nodal point of the lens not the center of the camera body otherwise your results will never be satisfactory.

Where exactly this point is, how to find it and how to fix this problem, will be covered in a later blog entry.
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Comments

  1. Old Comment
    CJ Swartz's Avatar
    I had no idea what a nodal point was, or why I'd want to know, but as always, your explanation answers both questions. Then just as I was wondering "how the heck am I supposed to know where it is on my camera?", you also explain that those answers will be forthcoming - well done.
    permalink
    Posted 01-07-2008 at 11:59 AM by CJ Swartz CJ Swartz is offline
  2. Old Comment
    Mining Art's Avatar
    Great Info Frank, Thanks
    permalink
    Posted 01-10-2008 at 11:00 AM by Mining Art Mining Art is offline
 

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