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Thread: Goerz Dogmar exposing unequally (different ISO) across aperture scale

  1. #1

    Goerz Dogmar exposing unequally (different ISO) across aperture scale

    So Iím seeing something that I donít think Iíve seen before. Iíve been testing a few papers for use in paper negative and direct positive processes. Twice now, Iíve been able to replicate the same behavior. Currently, Iím looking at Ilford Coldtone VC fiber paper (for direct positive but the development of the neg is the same as for paper negative essentially). Initially, I tested for speed using the step-exposure method. I used probably f8 or f16 when I did this. I got a paper speed of ISO 6 + 3 stops (I use a Pentax Spot meter that only goes down to 6 so I add the additional stops). This yields a good negative on this paper which would stand on its own as a paper negative and reverses well into a positive image. The problem now is that if I shoot the lens wide open, it is not following the same speed rating and the image area is totally exposed to black (negative) with no image detail. Complete total overexposure. Stop it down and use the same speed rating and the image forms correctly. Iím changing nothing else in the workflow. The only difference is the aperture used for the exposures. Has anyone experienced anything like this?

    And as far as the lens, Iím using a 10 3/4Ē Goerz Dogmar f4.5. The only thing I could think of was if the lens (itís an old lens) somehow has an aperture not conforming to modern standard. It doesnít have a typical f-stop scale and instead reads: f4.5, f5, f5.5, f6.8, f8, f16, f22

    Any insight would be appreciated.
    Francesco Fragomeni
    www.FrancescoFragomeni.com

  2. #2

    Join Date
    Aug 2018
    Posts
    347

    Re: Goerz Dogmar exposing unequally (different ISO) across aperture scale

    Except for reciprocity failure, nothing can explain this other than a mistake. Total black then I'm thinking removing dark slide before exposure (with shutter open.)
    Pushing exposure lever and putting dark slide in.
    Please show the difference.

    Sent fra min SM-G975F via Tapatalk

  3. #3

    Re: Goerz Dogmar exposing unequally (different ISO) across aperture scale

    Quote Originally Posted by Oslolens View Post
    Except for reciprocity failure, nothing can explain this other than a mistake. Total black then I'm thinking removing dark slide before exposure (with shutter open.)
    Pushing exposure lever and putting dark slide in.
    Please show the difference.

    Sent fra min SM-G975F via Tapatalk
    I was speaking with someone over on Photrio about this and reciprocity was brought up. I didn’t think of this. I think I’m just seeing some significant reciprocity for the paper that I need to work out. I was very controlled in the tests and the f-stop used was the only variable changed and there were no mistakes. That was why I took steps to replicate the results (I originally just though I must have pulled the dark slide and not realized it). So I’m now trying to figure out if there’s a way to determine a reciprocity factor I can apply to determine exposure as aperture changes (still trying to figure out how to do that) or I’ll just have to test at each aperture for effective iso and make a little chart.
    Francesco Fragomeni
    www.FrancescoFragomeni.com

  4. #4

    Join Date
    Jan 2019
    Posts
    619

    Re: Goerz Dogmar exposing unequally (different ISO) across aperture scale

    You donít mention exposure times but Iím assuming that at this low ISO you may be in the many seconds or even minutes regime?
    If thatís the case then maybe it is reciprocity. What I would do in this situation is to photograph a step wedge multiple times with different apertureís (or times) and see the exposure progression. With the wedge you can then measure (or eyeball with your eyes) an approximate stop correction as time increases.

  5. #5
    (Shrek)
    Join Date
    Mar 2011
    Location
    Montreal
    Posts
    1,994

    Re: Goerz Dogmar exposing unequally (different ISO) across aperture scale

    It's always possible the cells are in the wrong barrel. If someone took the Dogmar cells to mount in a shutter, and took the reciprocal cells from the shutter and stuck them in that barrel for safekeeping, your aperture scale could be off. Simple enough to check, measure your focal length and then the entrance pupil at several settings, if the division gives you a different number than what's on the barrel, then the cells are in the wrong barrel.

  6. #6

    Re: Goerz Dogmar exposing unequally (different ISO) across aperture scale

    At one time Goerz had their own scale. Look at the chart in this thread (note that modern f/stops are in the 1st column): https://www.photrio.com/forum/thread...f-stops.177675

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