Aperture placement

[Peter K]

Here is the schematicaly drawing.
Attachment 37920
To determinate the nodals points one has to focus the lens at a distant subject. Now the lens - not the camera - will be panned, moved perpendicular to the optical axis of the lens, and moved alonge the optical axis up to the point the image on the ground-glass doesn't moves during panning. (The position of the ground-glass has to be realigned to compensate the moving of the lens along the optical axis.) Now the plane perpendicular to the pan axis is the front nodal p point.

For determinating the rear nodal point p' the same procedure has to repeated with the rear cell of the lens.

Cheers

Peter

[swmcl]

Thanks Peter,

Your help is appreciated. I'll need a little while to digest this.

Cheers,

[Emmanuel BIGLER]

Hello all from France !

I agree with most that has been said above about nodal = principal cardinal points and the role and importance of the entrance pupil.
Cheeke in tongue : European solidarity here implies that I agree at 200% with Peter K, natürlich ! H und H' come from Hauptpunkte, Hauptebene !

Some additional remarks to the original questions
Is it important where an aperture iris sits along the barrel of a lens?

The correct placement of the iris determines
- residual aberrations
- and has a prominent importance for minimizing distorsion

In a symmetrical lens design if you do not place the iris at the centre of symmetry of the lens, you usually get distorsion.
In the Wollaston "landscape lens" meniscus single lens design, the shape of the meniscus (its radii of curvature) and the placement of the iris are determined by several trade-off between minimizing aberrations and distorsion.
http://www.optics.arizona.edu/Nofzig...ass9/fig22.jpg
The Wollaston lens is a purely academic case of little interest, except, may be, to show the kind of trade-off required in optical engineering, however it can be entirely computed by hand with a table of logarithms and a slide rule ; I suspect that some of us here only swear by the slide rule for their environmentall-friendly, zero carbon-dioxide routine computations

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For example look at the optical diagram of a classical dialyte lens (4/4) used in process cameras and still widely used by LF photographers today.

http://www.prograf.ru/rodenstock/ima...nar_scheme.jpg

I presume it should be placed at a 'nodal' point - which I assume is located precisely between the two innermost elements ...

Well, before speaking in public about notions in optical engineering, it is always good to precisely understand their meaning, otherwise the discussion can go in the wrong direction. I assume that after reading the previous posts, all our readers know what the nodal points N and N' are ( = H and H' principal points for a photographic lens in air), but I'm afraid that there is still a confusion floating around ... in air or glass about the various optical spaces encountered when light travels across a compound photographic lens.
The entrance space is air where the subject is located ; yes it can be water but let us start simply
Each time a ray crosses a diopter, you change from one optical space to another.
The iris is physically located in air but in a certain intermediate optical space different from the entrance optical space and different from the exit optical space.
The entrance pupil is the image of the iris as seen from the front of the lens. The entrance pupil is located in the entrance optical space. This is not the same optical space as where the iris itself is located.
Hence by definition of the entrance pupil, and taking into account that the proper rotation point for parallax-free panoramic stitching is the entrance pupil, and nowhere else, the iris can hardly ever been located at the same place as the entrance pupil, except in theory for an ultra-thin single lens element, or some very special lens design.

About parallax effects in panoramic stiching and the entrance pupil I had posted here an explanatory diagram...
http://www.largeformatphotography.in...0&d=1149151905

The relevant discussion started by (and starred by, of course) Leonard Evens is here
http://www.largeformatphotography.in...entrance+pupil

However in quasi-symmetric lenses like those in use in LF photography, the iris being located very close to the quasi-centre of the lens, it can be shown by simple ray-tracing considerations, that its images, the pupils, are located very close to the nodal points. Only very asymmetric designs like telephotos with long focal lengths have the entrance pupil located far from the nodal points.
Hence the entrance pupil is very often located very close to the entrance nodal or principal point !!!!!

Is it as easy as that ? Or is this an ideal - in which case there is some compromise if it is not centrally mounted ...

No, things are very, very complicated indeed.
For a perfectly symmetric lens design used at the 1:1 ratio, you should place the iris at the centre of symmetry otherwise you get distorsion. But even this rule of symmetry does not guarantee that you get a good design.
For example take a single lens element and try to use it for large angular field images of distant objects : in this case a symmetrical bi-convex lens element with the iris close to it is not the best design, one of the best designs for one single element is the strange Wollaston meniscus, a concave-convex single lens element, the concave surface facing the subject and with an iris placed in front, in air !

Now try to focus a parallel beam of light (like a laser), you do not care for a wide angular field at all, the optimization differs totally, you can place the iris wherever you like in front of the lens if the beam is really parallel to the optical axis, and it can be shown in classical textbooks that the best shape is an asymmetric bi-convex lens element, not at all Wollaston's meniscus.
http://www.thorlabs.com/newgrouppage...ctGroup_ID=900

In the 1:1 ratio with a symmetrical lens whatever design, distorsion is : zero, nil, nada, totally cancelled by symmetry. Plus : zero coma and zero lateral colour, which are nice bonuses for the engineer, and explain why quasi-symmetrical designs are so good and so often encountered in LF photography.

However if you need even a slight departure from this very restricted situation : 1:1 magnification ratio plus symmetrical lens design, think about a dialyte lens design used for distant objects, in principle you have to make compromises.
For a lens optimized for long distances, you should depart from a purely symmetrical lens design.

However, it is a matter of fact that many view camera lenses designed in the XIX-st and XX-st century (except telephotos and except the beloved asymmetrical Tessar design ; I am not aware of retrofocus lenses for the large format ?) are only a slight modification of a pure symmetrical design.
The miracle is that even if you do not respect the 1:1 magnification ratio, a quasi-symmetrical lens design can nevertheless offer you outstanding performance.
If you consider a telephoto or a retrofocus design, which are so common in 35mm and medium format photography, the iris has to be located after balancing many, many parameters, including residual aberrations and distorsion.
This is the art of lens engineering and cannot be explained in a few words.

However, since we mostly deal with quasi-symmtrical lens designs, yes in this case the iris is located very close to the quasi-centre of quasi-symmetry of the lens.

And for almost all non-symmetric designs used for LF cameras, the iris is located inside the lens, unlike the Wollaston lens !
Look at the tessar design, the iris is located in the middle of the lens.
The only exception I know for LF lenses is the "odd" Rodenstock Imagon(TM) where the lens, a cemented doublet, is located behind a very strange aperture that in France we call : "the infamous Imagon(TM) mashed potatoe machine grid "

[Mark Sawyer]

So! Its not as easy as I thought.

Firstly, I think I'm sorry I mentioned the technical term 'nodal' ... it seems it was a cardinal sin !!!

But seriously, thanks guys. Yes I have a lens in mind, an old magic lantern lens for a start. It may be a Petzval I don't know.

Where do I go from here? Must I get accurate measurements and then learn a computer simulation program ? It is beginning to be very complicated. I was hoping to cut it in two and be done ...

Rgds,

If you like the fabled "Petzval Signature" you're probably better putting it in the wrong place anyways! Putting it in the wrong place will just alter the field curvature (the Petzval isn't a flat-field lens anyways), and maybe add a little coma, which would just add to the charm for most photographers who like this sort of lens. It wouldn't hurt the resolution at the center, and wouldn't cause significant vignetting ulless you were waaaaaaay off.

Or if you want it "right", ask around about other Petzvals with a similar focal length, and ask the owners where their apertures are.

[Mark Sawyer]

Hey Mark, with all the digital stuff I have to teach, the Fellows want to shoot on film. They love it and we teach both captures. They're totally blown out with my 8x10 contact prints and love to shoot 35mm Motion Pic capture. I'm very proud to teach at the Conservatory that is considered #1 for Cinematographers on the planet. (And our faculty is modest too. ;-))

Sounds like a great place to teach! We do about 60% digital, 40% darkroom. Most of my kids want to do do digital when we're in the darkroom, and darkroom when we do digital, of course! A lot of the stuff is "high-school-ish", but fun if you can learn to appreciate it. With apologies for wandering off-topic, one of our shadows/lighting demos from last week:

[Mark Woods]

Nicely done! Is this one of your shots or a student's?

My class runs me ragged. They are curious about everything, as I'm sure yours does you.

[Mark Sawyer]

That was a demonstration print, made "by committee" as it were. Three students and myself, with me mostly making suggestions after a few earlier near misses, and them saying, "be quiet, we know what we're doing!"

[Mark Woods]

Oh man! I know what you're saying. I had the same experience on Thursday while we were shooting a beer pour at high speed with strobes. I had some wine :-) at the dinner break with my fellow teacher (does a lab while I do the theory -- sometimes the theory goes on stage).