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Thread: Positioning an iris/stop in a barrel lens...

  1. #1
    Name: ______William Booth
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    Positioning an iris/stop in a barrel lens...

    This is a bit of a cross-post between here and the Collodion.com forum, so I will start by posting my original question and then try to elaborate a bit:

    I have looked up and down the net seeking information about determining the correct position of a DIY iris/stop in a brass lens (petzval) barrel. *Most lenses I've seen that are cut for Waterhouse stops seem to have the slot (somewhat) centered between the front and rear lens groups, give or take 5-10% forward or back, but I've been warned multiple times against cutting a slot arbitrarily.

    The thing is, nobody has been able to tell me how to determine exactly where the stop needs to go. *I am going to start by constructing a washer stop (a ring with a hole in it) that I can slide into position along the barrel, but how will I even know when the stop is located in the right position? *Are there any telltale signs that I've got it located correctly or aberrations I should look for to tell me that I've got it located incorrectly?

    Does anybody here have any experience with this? *Because I'm feeling a little lost.

    Thanks!*
    Basically, I am finding that there are simply too many unknowns to try to calculate the front and rear nodal points of this lens, and that I'm going to have to use my eyes to produce any useful information. The lens is in Dallmeyer configuration, and it doesn't have an internal stop to clue me in as to the proper location of a stop.

    I have been told that the "Principal Point Separation" represents the distance between the two nodal points and is the space within which a stop will need to be located.

    I was hoping there was some formula I could apply to find the nodal points, but that seems not to be the case. *As I'm moving the stop, what are the characteristics of the image formed that I should be looking for? *Besides "looks sharp here", that is. *For instance, will the range in which I can move the stop without a discernable decrease in sharpness represent the location from the first to second nodal point? *Then should I just locate the stop between those two points?

    I found a couple of relevant posts on the internet about determining lens nodal points (for shooting panoramics), but there's some confusion/misunderstanding about the difference between the nodal point/s and the entrance pupil of a lens. *Either way, maybe these are a good place to start.

    http://teocomi.com/detect-the-nodal-...r-lens-how-to/
    http://www.thomas-schwenger.de/index...=sub_tt&pg=npe
    Anyway, the gist of the problem is that I want to place a washer stop and don't understand a damn thing about lenses or even the name of the proper iris location. Any help is really appreciated!!

  2. #2
    jp's Avatar
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    Re: Positioning an iris/stop in a barrel lens...

    I think the "nodal point" testing for pano work is completely different than the "nodal point" for lens fabrication. I haven't seen a clear explanation either, nor do lenses on the market clearly follow a an easily discernible pattern. In a simple 1-group meniscus lens, the iris works to vary the ratio of axial rays versus central rays going into the glass. Position doesn't seem to be super critical for this. It might be more important for depth of field control, I'm not sure.

  3. #3

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    Re: Positioning an iris/stop in a barrel lens...

    As far as I remember, the main aberration you need to control with aperture is geometric aberration. So: position your camera square to scene with some strong vertical or horizontal lines (like trees or building). Then move the washer stop between the groups until lines are as straight as they go (not bended inward or outward). The starting position could be in the middle between both groups.

    Marko

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    Re: Positioning an iris/stop in a barrel lens...

    I recently adapted a multi-element 18mm lens to M43 sensor and had to add a Waterhouse stop behind the rear element.
    Initially with the stop too far from the rear, the lens was sharp in centre, but edge sharpness was poor (blur rather than CA).
    Moving the stop to the correct position close behind the lens made a big improvement:
    https://www.box.com/s/f7b4205014069b6b453a

    I would like to get a lens design book, but if adapting a stop to an existing lens , maybe trial and error is appropriate.
    My next lens project will be a simple single element uncoated f/1.8 77mm with a waterhouse stop behind for f/4, f/16 etc.
    Hopefully that, with its uncorrected aberrations, will be both a portrait lens on 35mm and a wide angle on the Graflex.
    I read that a starting position for stop on such a lens is 1/5 of FL behind the element centre.

    For trial and error positioning, a sketch of the lens is useful:
    https://www.box.com/s/b9319930073aad14e911

    To get some adjustment of stop position, I machine an outer holder of acetal that is a sliding/press fit in the barrel.
    The actual stops are of brass which press into the acetal holder. I drill the f/stop to diameter and then counterbore with a larger drill until the thickness of the stop bore is as thin as I can get by eye. The stop is then sprayed with Krylon Ultra Flat Camo.
    I think such an arrangement could be used on a stop between elements with adjustable position.

  5. #5
    jp's Avatar
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    Re: Positioning an iris/stop in a barrel lens...

    wombat, for a single element lens, you might consider putting the iris ahead of the lens (like Kodak portrait, spencer port-land, struss pictorial, galli meniscus, reinhold wollaston). Provide a nice lens shade and protects the glass.

  6. #6

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    Re: Positioning an iris/stop in a barrel lens...

    jp:
    Good Idea, I had not considered. I will try to make versatile holders so they can slide in the barrel both ways.
    Test bed is M43 which i have adapted to a Pentax K bellows, the register range about 45~ 150 mm
    thanks.

  7. #7

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    Re: Positioning an iris/stop in a barrel lens...

    Before I discovered the simplicity of the single element Wollaston design, I wasted a lot of time chasing the chimera of replicating the petzval concept.
    A few minutes on wickepedia was quite informative.
    This link opened the door to simple lens design for me: http://en.wikipedia.org/wiki/Wollaston_landscape_lens
    Life got a whole lot simpler.
    Re-inventing the wheel is easier if you keep it simple: http://re-inventedphotoequip.com/Site/Home.html

    Reinhold

  8. #8
    Name: ______William Booth
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    Re: Positioning an iris/stop in a barrel lens...

    Quote Originally Posted by Marko Trebusak View Post
    As far as I remember, the main aberration you need to control with aperture is geometric aberration.
    Thank you Marko! It's amazing to me, given the recent popularity of these lenses, how little understanding/explanation there is of even the applied optics concerning them. I will use this as a starting point and let you guys know how it works out.

    Thanks to everybody who's participated in this discussion so far:

    jp489 for confirming the lack of clear explanations and confirming the misuse of "nodal point" in pano discussions,
    wombat2go for sharing your experience with positioning stops, and
    Reinhold Schable for sharing the interesting links.

    Much appreciated.

  9. #9
    renes
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    Re: Positioning an iris/stop in a barrel lens...

    Is the iris position also critical for symetrical anastigmat lenses like Protar, Dagor?

  10. #10

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    Re: Positioning an iris/stop in a barrel lens...

    Symmetrical versions of the protar and all DAGORs ex-factory are fitted with midway stops/irises. There is not much choice with Protars as the front and rear cells are almost in contact with each other!
    They must be moveable to some extent in symmetrical lenses as all convertable lenses (with the exception of the early Darlot casket set from the 1860's) have a single stop position in spite of very different focal length selections in the front and rear cells.

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