negative nodal measurement

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clippo
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negative nodal measurement

Post by clippo » Fri Dec 11, 2009 8:52 am

hoping to use my recently acquired Sigma 70-300 APO shortly and I was just looking into trying to find roughly where the nodal point of this lens is before calibrating it on my panosaurus.

I found a table on wikipedia which gives measurements for this lens but at 200mm and above, the measurement sugegsts the nodal point is beyond the rear of the lens.

Can anyone explain this? and if true, how to get around it?
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elf
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Post by elf » Sun Dec 13, 2009 2:12 am

The entrance pupil can be located in front of the lens, inside the lens, or behind the lens. It can be fairly stationary or it can move around dramatically when zooming or changing focus. What's even more entertaining is the exit pupil can be located in front of the entrance pupil :)

I don't have an optics degree, so I don't have an explanation.

The entrance pupil in my Olympus 70-300mm lens moves from near the front of the lens to 18+ inches behind the sensor which makes it very difficult to use on a pano head. I have had some success with handheld panos.

Can you link to the wikipedia article?

h-g-t
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Post by h-g-t » Sat Jan 15, 2011 8:03 pm

When I measured the distance of the nodal point of my Fuji S9500 from the tripod axis I had to do it again because I could not believe it!
However, I had done it correctly and it went from +78mm to -28 mm as I zoomed the lens.
It looked a bit funny on the pano head bacause it was entirely in front of the pano head axes when at full zoom.
Made it a bit difficult to set up the pano because I was reaching so far forward to see through the viewfinder.

When I tried measuring the modal point with my EOS/ Sigma 70-300mm I just gave up because it was nowhere near the tripod at maximum zoom. Just have to keep it for long shots.
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johnh
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Re: negative nodal measurement

Post by johnh » Mon Jan 17, 2011 5:04 am

clippo wrote:Can anyone explain this? and if true, how to get around it?
To explain this, you would need to examine a diagram showing the passage of rays through the lens, output by a ray tracing program. To see if it's true, all you need do is use your eyes. As implied by the previous post by elf, it is the location of the entrance pupil that is of interest, not the nodal point (which is irrelevant). With the lens mounted on the camera, stop it down to F/22 using the depth of field button and look into the lens while pointing the viewfinder at the back of the camera to a light, such as a bright window or the sky. You will see the bright disc of the illuminated entrance pupil and this will be seen to move forwards and backwards as you zoom the lens. To get the entrance pupil positioned at the pano head axis of rotation, you may have to reverse the top rail so that it points at the subject, or use a ring mount that's commonly used for mounting a telephoto lens onto a tripod.

John

h-g-t
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Post by h-g-t » Mon Jan 17, 2011 6:24 am

Found this article which, rather surprisingly, seems to say that the non-parallax point can move according to aperture!
http://www.janrik.net/PanoPostings/NoPa ... xPoint.pdf

Also found this :-
http://web.archive.org/web/200804071749 ... _Point.pdf

I'll keep calling it the nodal point though, too old to change.

The method I use which requires the tribrach and tripod from a laser level together with a metal bracket bolted to a piece of wood about 20-24mm wide.

Place the wood into the tribrach and mount the camera on the bracket.
Find a comfortable seat in your living room facing a window and line the camera up with part of the window frame (or a string hanging from the curtain rail).
Swing the camera from side to side and watch how the background outside moves relative to the window frame.
Slide the wood along the tribrach and repeat.
If the background moves faster then you are moving away from the nodal point so slide the wood in the opposite direction.
Repeat until the background stops moving at all, no matter how far round you swing the camera.
The nodal point now lies over the centre of the tripod so you can measure where that lies relative to the camera or just mark the wood and measure later.
Repeat for different lenses or different focal lengths if you are using a zoom.

Use a longish bit of wood. I was surprised to find that my Fuji S9500 sat well in front of the tripod at maximum zoom!

From the article above it seems that you might also have to repeat at different apertures - must try that and see.

"The entrance pupil in my Olympus 70-300mm lens moves from near the front of the lens to 18+ inches behind the sensor which makes it very difficult to use on a pano head. I have had some success with handheld panos. "

If you use a long bit of wood in a laser lens tribrach you can move the camera pretty far. Might have to hold on to or weight the tripod so stop it falling over!
Inebriated with the exuberance of his own verbosity, and gifted with an egotistical imagination .....

elf
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Post by elf » Tue Jan 18, 2011 5:07 am

You'll also want to repeat the measurement for different focus points as well. Fortunately, locating the entrance pupil to the nearest hundreth of a millimeter isn't necessary. Unless you're doing macros like this:
http://img.photobucket.com/albums/v649/ ... ce_fsb.jpg

Photosynth: http://photosynth.net/view.aspx?cid=3c6 ... 46feefa9c5

I think the Sigma (my 70-300 is a rebranded Sigma) can be used effectively for hand held panoramas, but isn't practical for panoheads because of the large movement of the entrance pupil. For handheld panos, set the zoom and focus point, then locate the entrance pupil by looking in the front of the lens. The apparent position of the diaphram is the entrance pupil. The only hard part is learning a good handheld technique for rotating around the entrance pupil.

(edit) I should have read johnh's response closer:) It will be hard to use the longer focal lengths of this lens on a pano head, but the shorter ones should work fine.

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