Nodal Points on 90-280mm

[tthorne]

Advertisement (gone after registration)

I was just wondering if anyone here has measured for the nodal point at any focal length on the 90-280mm. Usually I have no problem doing this with wider lenses, but I am having a difficult time with my normal dual light stand method. Particularly looking for the nodal point at 280mm. 

 

[scott kirkpatrick]

Sorry, I can't help.  In fact, I didn't know you could measure a nodal point position experimentally.  Could you give a reference or two?

[Ingo]

I've done this only with the 24-90 wich was no challenge. My Novoflex VR-Slim is not sturdy enough for the SL + 90-280 so I skipped this exercise. 

What is your use case for a 280mm Panorama? Brenizer or Gigapixel?

[tthorne]

I am measuring it for the purposes of shooting a panorama. I think that "nodal point" is actually a misnomer in this case and I am looking for the exit pupil or the point of no parallax. There are some videos online demonstrating the method that I use. Essentially I mount the camera/lens on the tripod and place one nightstand directly in front of it about 65" away and another one directly behind that about 250" away. Then I watch on live view and swing the camera back and forth. As it moves left to right the rear light stand becomes more visible. So then I adjust the rail moving the camera forward or back until the two light stands stay in line despite panning left or right. 

 

This has always been easy, but I have never done it for a focal length this long. 

Edited August 27, 2017 by tthorne

[Ingo]

65'' is quite near for a 280mm lens. So the parallax effect will be (very) much more visible than on a 24-50 mm lens (the typical focal length for panoramas and used in the online tutorials). Will you have an object in your panorama that is so close? Otherwise I propose to increase the distance to 250'' for the nearer light stand and 400'' for the second one. 

[gvaliquette]

tthorne,

 

Technically, what you are looking for is the "entrance pupil", not the "exit pupil", the point around which the lens-camera unit must rotate for parallax-free stitching.  You can get a first approximation by looking through the lens (without a body behind it) and make a "guesstimate".  I agree with Ingo that your test target is too close.  Go outside and plant a stake in front of a much further (preferably) small tree to do your test.  You are fundamentally correct in your test technique, but your targets are too close for a 280 mm lens.

 

Guy

[tomlianza]

Advertisement (gone after registration)

Lenses have two nodal points.  The location of the rear nodal point is, by definition, 1 focal length from the image plane at infinity focus. This is very different than the back focus of the lens.  It is important to note that this means that the nodal point might be deep inside the camera body for a wide angle (retro focus) lens, or way outside the body in the case of a true telephoto lens.  The entrance pupil of a lens has nothing to do with the Nodal Points unless you are using a very symmetrical design (such as a double Gauss ) where the stop is placed symmetrically near the nodal points.  For Pano images, you need to rotate around either nodal point, but the rear nodal point is the most easily measured.  Wide angle lenses for SLR cameras are a particular challenge to design because of the location of the nodal points outside of the physical lens.  In some of these lenses, the "rear" and the "front" nodal points are actually reversed due to the design constraints for back focus....

[01af]

For panorama images, you need to rotate around either nodal point, but the rear nodal point is the most easily measured.

That's wrong. Instead, it's the no-parallax point that must be rotated around. The no-parallax point is where the optical axis and the entrance pupil intersect.

 

 

You can get a first approximation by looking through the lens [...] and make a "guesstimate".

Exactly. Close the aperture to any f-stop besides the maximum, so you can see the diaphragm blades. The entrance pupil is where the diaphragm blades appear to be when looking into the lens from the front. This apparent position can be—and usually is—different from the diaphragm's physical position, so looking at the lens cross-section diagram won't help. (And by the way, the exit pupil is located at the diaphragm blades' apparent position when looking from the rear, but I digress.)

 

These things have nothing to do with the lens' nodal points.

Edited August 30, 2017 by 01af

[saxo]

I've done this only with the 24-90 wich was no challenge. My Novoflex VR-Slim is not sturdy enough for the SL + 90-280 so I skipped this exercise.

 

Thats interesting, as Novoflex suggested VR-slim when I asked for a pano system. I thought it may not be sturdy enough. I go for RRS. But its hard to find a dealer.

[Ingo]

Thats interesting, as Novoflex suggested VR-slim when I asked for a pano system. I thought it may not be sturdy enough. I go for RRS. But its hard to find a dealer.

 

Of course it will not break but in my opinion it's not built for telephoto lenses. This is okay because I shoot panoramas with 35mm.

 

The best (and only?) dealer for RRS in Germany is www.augenblicke-eingefangen.de

[Andrew Gough]

This article explains all.

 

http://www.janrik.net/PanoPostings/NoParallaxPoint/TheoryOfTheNoParallaxPoint.pdf

[tomlianza]

Hi to all, 

I have downloaded the suggested article and read it and many of the references in the article.  The article describes the theory of a "No-Parallax" point in Panorama photography.  The questions are : Is "No-Parallax" imaging really desired and did the author actually prove his point?  The answer to the first question is "maybe" and to the second question: "no".  If you take the time to read and understand the paper you will see some obvious issues.  First, the author demonstrates the shifting perspective by moving the aperture.  This is a very good demonstration.  At that point in the article, the author could have rotated camera around his new entrance pupil aperture and showed that the perspective did not change and the point would be proven (at least by example) .  It is not until the end of the article (page 17 ) that he describes an experiment that is "weird" and cautions that if the location of the limiting aperture changes across the image width, the  lens will "not have a single no-parallax point, but rather a collection of "least-parallax" points that vary with angle from the optical axis".  

 

Now let's talk about the desirability of "no-parallax" imaging.  If we can imagine a scene of two fences stretching across the horizontal field .  One fence is in the foreground and one fence is further back.  Both fences are axially aligned so when we look at the center posts, they are completely aligned. As we move our head left and right we find that the fence images start to diverge.  This is what the author seems to be referring to as "parallax". It is the result of the tangential mapping of the field as we change our angle of view.  So the question is, do we wish to maintain the perspective of a human scanning the horizon or do we wish to simulate riding a path parallel to the fence and stitch images together to form a parallax free image? Once again, what is the author trying to achieve?  

 

The author briefly mentions the design of panoramic cameras with a curved focal plane, but fails to mention a very important distinction.  The author notes that the angular field of view in a no-parallax case is given by the relationship: theta = 2* arctan (d/2f), where d= the sensor dimension and f the focal length of the lens.  In the special case of a panoramic camera, the angular field is maintained so there is no tangential effect registered on the focal plane.  This is by no means no-parallax imaging.  If you looked at our two hypothetical fences, they still would not appear to be aligned.  The relative spacing between the fence posts would be physically correct, but they would look odd because of the f-theta relationship.  The limiting format of a panoramic camera is typically 1:2.5 due the limitation of the film mounting and the fact that the images start look highly distorted at greater aspect ratios.  

 

One point that the article seems to gloss over is the fact that NO MATTER which point you rotate about(Nodal point or Entrance Pupil), the image plane will be tilted relative to the original object plane.  Under these conditions, a number of things can happen which are affected by the aperture stop. The author talks about the telecentric stop and correctly points out that with a stop in this configuration, the size of objects does not change appreciably with focus.  In a normal photographic system,  both the size and focus of the image change to either side of the axis in the opposite direction and that is not good.  The only way to counteract this is to stop the lens way down and let the depth of field take care of it.  This is in fact what the author recommends but to limit physical vignetting, not increase depth of focus.  The article does mention Panorama Tools and it seems that this might be a useful tool to experiment with.  While the article was not technically compelling, the concept is so I will give it try, but I am still not sure that no-parallax stitching is going to provide the "best" image.  So, it is off to download Panorama Tools and do some imaging....!

[01af]

The questions are: Is "No-Parallax" imaging really desired and did the author actually prove his point?

The answer to the first question is "yes," and to the second, "yes."

[Tancho]

I've done this only with the 24-90 wich was no challenge. My Novoflex VR-Slim is not sturdy enough for the SL + 90-280 so I skipped this exercise. 

What is your use case for a 280mm Panorama? Brenizer or Gigapixel?

 

we could have your values for 24/90

thanks

[dgktkr]

we could have your values for 24/90

thanks

 

Leica publishes the location of the entrance apertures: https://us.leica-camera.com/content/download/128358/1596835/version/7/file/Datenblatt_Vario-Elmarit-SL_24-90_e_1.pdf

 

For the 90-280, it looks like a typo for the position of the entrance pupil at 280mm (https://us.leica-camera.com/content/download/131428/1692725/version/4/file/Datenblatt_APO-Vario-Elmarit-SL_90-280_e_1.pdf): -21.1mm??? I doubt it.

 

dgktkr

[Tancho]

Leica publishes the location of the entrance apertures: https://us.leica-camera.com/content/download/128358/1596835/version/7/file/Datenblatt_Vario-Elmarit-SL_24-90_e_1.pdf

 

 

 

 

"Focal length 24 mm: 103.9 mm; focal length 90 mm: 92.6 mm"

Distance to the baionette, or to the front lens ?

Edited January 16, 2018 by Tancho

[gvaliquette]

 

"Focal length 24 mm: 103.9 mm; focal length 90 mm: 92.6 mm"

Distance to the baionette, or to the front lens ?

 

 

... or focal plane.

[FlashGordonPhotography]

Usually from the focal plane.

 

Gordon