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Thread: Aligning BOSSCREEN to film plane

  1. #1

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    Aligning BOSSCREEN to film plane

    Iím looking to reconcile two contradictory findings in my pseudo-scientific endeavor to nail down focus on the BOSSCREEN ground glass that my camera came with.

    Iím presenting my method and if someone can spot my mistake I would be very grateful.

    I was shooting slides (E100G) for the first time and was just a little disappointed with the detail on the horizon of a landscape scene. The shot was made racked against the infinity stop of a Linhof Tech V. Now, I have checked the infinity stop against the GG painstakingly in my endeavor of projecting a laser through the rangefinder. My new paranoid hunch was that maybe the GG was not co-registered with the film plane. Now, the previous owner cannot be asked how he installed it, because he fell victim to cancer (which is why I have the camera).

    Since I had a big load of old Xray film I commenced to photograph a table top with a newspaper on it at 45į



    I focussed on a particular line (thin line under "Asset Management" at 10cm on the paper ruler) but the focus on the developed sheet seemed to be 1 cm out.

    Now, 3 columns of text were (roughly) 18cm wide on the paper and 7cm on the film giving a magnification of 1/2.6. The other calc 150/(545-150) found the same ratio of 2.6.

    I calculated with 10mm offset "(10mm * (sqrt(2)/2)) / (2.6**2) = 1mm", and hence got 1mm film deviance from the ground glass.



    Also in the repeat.



    Then I lifted the BOSSCREEN from its frame and found no metal shims (which are supposed to be there on a Technika) but two 0.9mm thick rubber-like stacks of electrical tape instead of shims.


  2. #2

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    Re: Aligning BOSSCREEN to film plane

    Since the 0.9 mm were agreeing with my estimate I peeled them off and fixed the screen directly onto the metal flange with the four set screws fully retracted. I took another test shot and focus seemed to match now.



    However, and now comes the problem, today I took more test shots comparing the old infinity stops to my new ground glass setting but things at infinity didnít get better, they got worse. For some reason what matched at close-up distance didnít match at infinity.


    So, today I wanted to try a different route to measure the distance of the film plane and Bosscreen wax layer each. Just breaking out the calipers would not have given the right accuracy obviously. And I have no advanced tools like micrometers. And in the BOSSCREEN the image layer is in the center of a glass sandwich preventing purely mechanical measurements.

    But, I have also inherited an old 40x Zeiss stereo microscope:



    I dropped a Durst enlarger glass on top of the GG frame to measure the focus difference between a felt pen mark on the underside of the glass pane



    and the wax layer inside the screen.


  3. #3

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    Re: Aligning BOSSCREEN to film plane

    Then I could use my callipers to measure from these two defined spots




    to make a measurement with Ī0.05mm accuracy (by reading the nonius at right angles)



  4. #4

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    Re: Aligning BOSSCREEN to film plane

    The vertical focus offset between the lower and the upper level would give me the flange distance from the GG




    And the same story with the film holder




    I measured everything 3 times and calculated both flange distances and got 4.8mm for the film holder (the specification standard seems to be 3/16 inch, so 4.76mm) and got 3.9mm for the new "naked" position of the BOSSCREEN wax layer. A difference of 0.9mm!!!

    So it appears, after all, that the original rubber shims were correct when measuring (opto-)mechanically. But on my table top newspaper test there was a clear problem.

    If anyone of the wonderfully knowledgeable people here can spot my error please help me find it.

  5. #5
    Maris Rusis's Avatar
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    Re: Aligning BOSSCREEN to film plane

    It could be an optical path difference problem caused by the glass in front of the wax layer. Light passing through glass appears to travel a greater distance than if there was only air there. Optical pathlength is geometric pathlength multiplied by the refractive index of the medium the light is passing through.
    Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".

  6. #6

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    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Maris Rusis View Post
    It could be an optical path difference problem caused by the glass in front of the wax layer. Light passing through glass appears to travel a greater distance than if there was only air there. Optical pathlength is geometric pathlength multiplied by the refractive index of the medium the light is passing through.
    That is an interesting point. However, didn’t I already compensate for this by measuring through the glass in question? Or does the refraction by the inner pane of glass depend on the incident angles? Does this explain the difference between 0.75 m and infinity?

    That would be a flaw in the Bosscreen itself, no less.

  7. #7

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    Re: Aligning BOSSCREEN to film plane

    First, contact a Linhof service center and get a set of the correct shims. They are the same as the current ones for the MT.
    Second, Do you have the Boss Screen for the Linhof?

    If not, find one, if you can or just get a new enhanced screen from anyone else.

  8. #8

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    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Bob Salomon - HP Marketing View Post
    First, contact a Linhof service center and get a set of the correct shims.
    I can ask, but the last time I wrote email to linhof.de they quoted 25 EUR for an infinity stop. I’d rather figure this out myself. That is half of the fun in LF for me.


    Second, Do you have the Boss Screen for the Linhof?
    No, it says BOSSCREEN HOLLAND - PLAUBEL. Are there differences in the stand-off?

    In the end I don’t want to buy anything or pay someone. I just want to know the best method to co-register the screen I have.

  9. #9

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    Re: Aligning BOSSCREEN to film plane

    Something that just occured to me: Would the colour of light (let’s say 6500K vs 3200K) affect the results of my newspaper tabletop test? Would it contribute that much? It’s noteworthy that I was using blue-sensitive Xray film in both cases.

    I got this idea because some lenses have a red dot to focus on (behind ∞) when using IR film.

  10. #10

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    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by towolf View Post
    I can ask, but the last time I wrote email to linhof.de they quoted 25 EUR for an infinity stop. Iíd rather figure this out myself. That is half of the fun in LF for me.

    Yes. Since the image forming layer is the wax between the two sheets of glass there are major difference. So your best course is to get the camera back set back to where it belongs, on the shims with the adjusting screws properly adjusted and install a proper screen.

    No, it says BOSSCREEN HOLLAND - PLAUBEL. Are there differences in the stand-off?

    In the end I donít want to buy anything or pay someone. I just want to know the best method to co-register the screen I have.

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