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Thread: Focusing error

  1. #41

    Join Date
    Sep 2007
    Posts
    644

    Re: Focusing error

    I think its sharpest left side -minute swing- and focused only a few inches in front of the window trim
    the photo on wall looks sharp but its the nearest to window edge that is sharpest and I don't think its as sharp as the nearest to window edge of the mirror
    the mirror is maybe 2.5 inches in front of the trim
    trying to shoot cobwebs probably off by 4in

    I think you just missed focus at 5.6 ..and have some swing somewhere

  2. #42

    Re: Focusing error

    Quote Originally Posted by shadowleaves View Post
    Ok, here comes THE answer....

    <snip>

    I'll leave that up to you guys to decide whether you want to drill a hole in the fresnel or to remove it completely or to live with the focus error.
    Seems to me that there is a fairly simple solution to this, easier than drilling a hole thru the fresnel. A rectangular shim that is the same thickness as the fresnel could be attached to filmholder/roll-film-back/digital back/whatever to compensate for this apparent problem. Or just inserted between the film holder and the back of the camera.

    My Horseman VH-R uses a similar technique with its Polaroid back to compensate for a similar spacing issue. The spacer is a standard Horseman SKU that ships with the Polaroid back.

  3. #43

    Join Date
    Sep 2006
    Location
    Frisco, Texas
    Posts
    143

    Re: Focusing error

    The more I look at your window picture, the more skewed it looks to my octogenarian eyes. I think the name given this by LF people is yawing or maybe perspective error.
    I think you are too careful to be using defective equipment but since you are shooting wide-open, try the following: get your hands on a good 35mm. or 120 rangefinder camera; stand where you think you will be sighting head-on dead center to the window frame with the film plane parallel to the window frame-glass: see if the range finder keeps EXACT FOCUS-DISTANCE setting if you shift from right side of window frame to left side of window frame, back & forth. The old Contax 35mm. RF cameras would register a change in distance focus from one side of an electric switch plate to the other side (80mm.) because the wall is not so flat that both sides are equidistant from the camera lens and/or I was not holding camera to result in parallel film plane to switch plate. Only by stopping down could I get enough depth of field to have entire switch plate parallel to film plane in the eyes of the lens; now magnify that to an entire window frame, perhaps an old window frame that is not flatly parallel to your film plane no matter what. Can you be certain that you are in dead center to the window plane? Stop down: if you were doing landscape work, you could make sure not to rack lens all the way back to infinity which could result in a corner or side being minutely closer to film plane than infinity; you do not have this leeway in close-up LF with lens wide open. There are reasons that Linhof takes such pain to have camera front standard parallel to film plane.
    Bob & are are not related but arrangements can be made.
    Bernie

  4. #44

    Join Date
    Sep 2009
    Posts
    10

    Re: Focusing error

    how big of a difference is the brightness on the ground glass, comparing a chamonix with fresnel and chamonix without?

  5. #45
    3D-Stereo-Aeropanoramas
    Join Date
    May 2004
    Posts
    186

    Re: Focusing error

    Quote Originally Posted by ic-racer View Post
    Shim your ground glass back (away from the lens). You can use trial and error, or a quick calculation. Did you focus on the window and the frame on the wall is in focus? If so your error can be estimated by the 1/p + 1/x = 1/focal length equation.
    So if you focused at 5000mm and the film showed focus at 4500mm from the camera, the ground glass is off by around 1.3 mm (if you are using a 150mm lens)
    what is p and x?
    Have the very same FL and Zone Z1 is too short. have all data only want to prove with this formula.
    www.stereopan.org
    3DStereo-Aeropanorama-Jungfraujoch

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