How to find the nodal point of a lens?

[Marco Milazzo]

I have a panorama project in mind. I'd like to pan the view camera from the nodal point of the lens. Is there a way to calculate the nodal point precisely, or would estimaing the middle -- i.e., the point between the front and back glass surfaces -- be close enough?

TIA

[Mark Woods]

An easy way to find the nodal point of the lens is to put two vertical elements in front of the lens. Move the camera back or forward until the two verticals don't move in relationship to each other.

[Marco Milazzo]

An easy way to find the nodal point of the lens is to put two vertical elements in front of the lens. Move the camera back or forward until the two verticals don't move in relationship to each other.

I'm probably dense (most people say so), but I don't understand. How does that tell me where the optical center of the lens is?

[Lachlan 717]

Try pivoting around the iris in the shutter, assuming a) the lens has a shutter, and b) the lens isn't a telephoto design.

[Tim Povlick]

Hi Marco,

Here is a website that shows images / text of what Professor Woods was describing.

http://dgrin.smugmug.com/gallery/2114189

Good Luck & Regards,

Tim

[BradS]

if it is a schneider lens, you can also get the information from their data sheets - which they have very conveniently posted on line.

[Emmanuel BIGLER]

How to find the nodal point

Hello from France and welcome to the Entrance Pupil Club !
The proper position of rotation of the whole camera for panoramic stitching is the entrance pupil of the lens and nowhere else.

It is somethimes specified in manufacturers's data sheet (e.g. Zeiss medium format lenses, Schneider large format lenses, ...) but the best is use an experimental procedure as explained by Alain Hamblenne (in English and French)
http://www.outline.be/quicktime/tuto/
http://www.outline.be/quicktime/tuto/TheGrid.pdf English tutorial

http://www.outline.be/quicktime/tuto/LaGrille.pdf
Same in French.

More readings about "Why is it the entrance pupil and nowhere else" a favourite discussion on our forum ! ;)
http://www.largeformatphotography.info/forum/showthread.php?t=49706&highlight=pupil+entrance

A geometrical proof (for those who like geometrical optics and hand-crafted ray tracings ;) )
http://farm2.static.flickr.com/1156/827305231_a38019c2f8_d.jpg
http://farm2.static.flickr.com/1122/827305245_c7bc05f5d2_d.jpg

[Bob Salomon]

If this is an outdoor project and the scene is more then 150 yards from the camera then you don't need to rotate around the nodal point. If you are doing interiors then you do.
Mark is right, that is the simple way to find the nodal point. Two vertical lines and then rotate the camera so one does not rotate in relation to the other. If they do move the point of rotation towards or away till they do not move when you rotate the camera.

[Marco Milazzo]

Thanks all, for many good answers.

[Lynn Jones]

It is simply adequate to rotate aound the lens aperture, that has to work or the aperture would cause a distortion. I have done this successfully for several decades.

Lynn

[Paul Fitzgerald]

How did the old Cirkut pano cameras work? They did not rotate around either nodal point.

[Emmanuel BIGLER]

It is simply adequate to rotate around the lens aperture

Yes, exactly, but not around the physical location of the aperture, but instead around the location of its image as given by the first half of the lens, looking from the front side of the lens. In some lenses like the apo ronar, the distance between the iris and the pupils is very small, about 1% of the focal length.
"The image of the iris seen from front" is simply the definition of the entrance pupil and this is the right place to rotate the whole assembly (lens + camera + detector). for panoramic stitching afterwards.

How did the old Cirkut pano cameras work? They did not rotate around either nodal point.

I do not know the details of the Cirkut camera, but if its principles are similar to what I know in other panoramic cameras like Noblex or Seitz panorama cameras where the lens moves with respect to a fixed film holder, hence the rotating point of the lens should in principe be the rear or exit nodal point of the lens usually named N' = H'.
If the Cirkut works differently, i'd be interested to know how the Cirkut works.

The two problems are different.
In panoramic stitching you want to avoid unwanted parallax effects that could be detrimental to the stitching process. So the condition you are looking for is that objects that look aligned on image #1 are still aligned on image #2 after rotating the whole camera for shot#2. The proper place for this is the entrance pupil.

In panoramic cameras where the lens rotates with respect to a fixed film, film being fixed with respect to the landscape, you need to avoid image blur during the lens movement. Only a thin strip of the image is exposed at a time, a slit moves just in front of the film in synchronism with the lens.
For far-distant objects, the rotating point minimizing blur when a lens rotates is the exit nodal point N' or H'.
This is valid only for far-distant objects, see the attached pdf diagram for the explanation.

For example, a very special (hypothetical ?) panoramic camera operating at the 1:1 ratio with a symmetric lens should probably rotate the lens around its center of symmetry to minimize image blur during the rotation process.

Note that the Alpa Rotocamera (was actuallly made by Seitz AG, not by Pignons SA) has a special operating mode ; the whole camera rotates, i.e. the lens is fixed with respect to the camera body, but the film moves in the proper direction and at the proper speed so that the film is fixed with respect to the landscape during the movement. This is in fact equivalent to the Noblex system, but for an observer attached to the film ;-)
In this case, the rotation point should also be the rear nodal point of the lens.
Actually in the Alpa Rotocamera which I have examined a few times in a friend's collection,, the lens is a 75mm -6.8 Grandagon-N, which is almost symmetrical, hence the pupils are located at the nodal points, and are very close to each other. So it is not easy without a precise measurement tool to know exactly where the engineers have decided to rotate ;)