Vibration or not

Discussion in 'Large Format Cameras and Accessories' started by rmolson, Oct 2, 2009.

  1. rmolson

    rmolson Member

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    Test of film plane vs ground glass

    I thought I had a problem with vibration, which I might still have. But a shot out doors of the street with an island divider tipped me off to a more serious problem. I was focused on the center of the island a good 75 yards away or infinity with a 150mm lens
    . But when I developed my negative the center was soft but on the side of the negative was the image of a parked car beside the street and closer to me and it was sharp. So I set up a test indoors with a ruler on a diagonal and focused on the center at a shallow f/8 opening ,the lens is f/5.6
    As someone said had I checked to see if the ground glass was mounted correctly I ran several tests.First test was with the present replacement fine ground, ground glass as is on the camera.
    The Second test I removed and reversed the glass and as expected it was a total wash.
    The third test I replaced the glass with the original coarse ground, ground glass
    Exposure was with a Vivtar 283 set at f?4 an equivalent 1/500 or higher flash duration, enough to rule out vibrations
    Both the first and third test shots showed the same problem
    Target a 40 cm ruler on diagonal Camera 40 inches from target ,150mm lens Both original and fine ground glass show same problem .The sharpest image is 2cm’s past the target point , the center of the ruler or 20 cm.
    Both the ground glass image and the aerial image agree
    If I interpret this correctly it means the film holder plane is behind the ground glass plane? And the ground glass needs to be shimmed, or is it the other way around ? Which would be problem with a metal camera.?
    They are Riteway holders in relative good condition
     
  2. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    First off, the matte side of the groundglass should be facing the lens. Is that the case?
     
  3. Vaughn

    Vaughn Subscriber

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    Second off, what camera/back are you using -- some, I believe, were designed to use a fresnel screen in front (towards the lens) of the GG...rare, but possible. So if there is no fresnal and there was suppose to be, focusing would be off by the thickness of the fresnel.

    I bought a used Zone VI 8x10 from Midwest Photo that had a fresnel in front of the GG, and since I don't like fresnels, I immediately removed it. Much later I got worried about my focusing, but it ended up that I did the right thing as the fresnel was in the wrong place when I got the camera. Perhaps that is why the camera was in such good shape and for sale -- the previous owner might not have been able to get sharp negs and sold it!

    Vaughn
     
  4. DannL

    DannL Member

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    I recently enountered this same problem with the 8x10. I need to go to the hardware store and buy some aluminum flat stock and attach several strips as permanent shims.
     
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  5. RalphLambrecht

    RalphLambrecht Member

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    Do what David said. Then, if you don't have a fresnel lens or if it is on the film-side of the glass, test ground glass vs film plane location as shown in the pictures attached. That should tell you what is wrong. If you, however, have a fresnel lens, and it is on the lens-side of the glass, things are bit more difficult.
     

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    Last edited by a moderator: Oct 2, 2009
  6. Vaughn

    Vaughn Subscriber

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    Hey, Ralph, I have used a ruler to double check the distance to see if they are the same -- but I like you quick and easy toothpick method!

    Vaughn
     
  7. rmolson

    rmolson Member

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    The camera is a Bush model D press view all metal and the is no fresnel lens. the ground glass sits flush on a metal bed and yes the ground side is towards the lens.The best I can figure is that the ground glass needs to move forward a tad or the back the holder rests against needs shimmed just how you get the back off a Busch is a mystery.
     
  8. rmolson

    rmolson Member

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    vibration

    I measured the distance from the camera frame to the ground glass with a vernier scale instead of the toothpick method and got a reading of 6.1cm
    Then I put in a holder with a sheet of film and got a reading of 6.2 cm.
    Which if I interpret it right it means that the ground glass could be shimmed by 0.1 cm ,which is doable. Does that makes sense?
     
  9. RalphLambrecht

    RalphLambrecht Member

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    rmolson

    Are you sure you're talking about cm and not mm?

    A small misalignment of the ground glass is compensated through the depth of focus, which is the zone of reasonable focus around the ground glass, similar to the zone of reasonable focus around the object, called depth of field. What is different between the two is that the depth of field increases with distance and the depth of focus decreases with distance. In other words, for far-distance photography and wide-angle lenses the exact position of the film plane is critical.

    Here is a quick table showing the accuracy required for three different magnification levels:

    Depth of Adequate Focus around the film plane [mm]
    -------------Scale
    f/stop ------1: >10------1:2-----1:1
    5.6---------1.0-----------1.4------2.0
    8-----------1.4-----------2.0------2.8
    11----------2.0-----------2.8------4.0
    16----------2.8-----------4.0------5.6
    22----------4.0-----------5.6------8.0
    32----------5.6-----------8.0------11.0
    45----------8.0-----------11.0-----16.0
    64----------11.0----------16.0-----22.0

    Consequently, f/8 gives you a bit more than 1 mm depth of focus around the film plane, unless you are focusing very close. I think, ideally the film plane position should be within a tolerance of 0.1 mm. The manufacturing tolerance for a 4x5 film holder is +/- 0.02 mm by the way.

    After you've made your adjustments, and you want to verify the results, remember to do so at infinity, wide open and with as short as a focal length as you can find, because that's where depth of focus is most critical.
     
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  10. Mike1234

    Mike1234 Inactive

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    If the GG was reverse... with the matte side facing the outside wouldn't he be focusing beyond infinity rather than closer?
     
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  11. rmolson

    rmolson Member

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    the veiner scale reads in cms and the difference betweenthe two reasding is .1 When the lens is focused on infinity and stoppped downto f22 the image is soft, which lead me to checking everything else.The Busch is at least 50 years old and may have at one time been shimmed,although there are no signs of it Going to find something about 1 mm thick and give it a try
     
  12. Richard T Ritter

    Richard T Ritter Member

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    I see this a lot on the cameras that come in to be worked on. Sometimes I get then in with a Fresnel that is frosted and a sheet of frosted glass together. Complaint is "I having a hard time focusing the camera".

    If the complaint is "it is not photographing the focusing point as I had it focused when photographing the image wide open". The ground glass or the Fresnel screen with the frost focusing surface are in backwards.

    If you think the focusing is out best to send it out and have it checked. S K Grimes and myself do this service.

    Sometimes a leaky bellows will show up as a focusing problem.
     
  13. RalphLambrecht

    RalphLambrecht Member

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    This would make for a good discussion in a different thread, because I believe there are two different legitimate ways to combine ground glass and fresnel lens. However, rmolson already stated that he is not using a fresnel at all.