Internal ND filters on the XH-A1 affecting focus

Discussion in 'Professional Video Production' started by Jacques E. Bouchard, Jul 30, 2008.

  1. It's the other way around. First I focusing at f/9.5. The image is sharp,
    but still a little bright. So I throw in the ND filter, and go to f/3.7.
    Suddenly I can't focus anymore.

    Why is that?


    jaybee
     
    Jacques E. Bouchard, Aug 3, 2008
    #41
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  2. "Jacques E. Bouchard" wrote...
    "For a given f-number, increasing the magnification, either
    by moving closer to the subject or using a lens of greater
    focal length, *decreases* the DOF; decreasing magnification
    increases DOF. For a given subject magnification, increasing
    the f-number (decreasing the aperture diameter) increases
    the DOF; decreasing f-number *decreases* DOF"
    http://en.wikipedia.org:80/wiki/Depth_of_field
     
    Richard Crowley, Aug 3, 2008
    #42
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  3. Can't answer that if you refuse to do the *inverse* experiment
    as suggested. It is called "experimental process" or "differential
    diagnosis" if you wish.

    You appear to be refusing to do the experiment.
    You appear to be refusing to answer the other questions.

    Why is that?
     
    Richard Crowley, Aug 3, 2008
    #43
  4. How does that go against what I just wrote?

    Focused at 3,000 feet the DoF is about 500 ft to infinity at f/3.6, and
    about 230 ft to infinity at f/9.5.

    I think it's safe to consider both values "huge".


    jaybee
     
    Jacques E. Bouchard, Aug 3, 2008
    #44
  5. Richard, read the post before replying to it.

    I. Can't. Focus. At. 3,000. feet. With. The. ND. Filter. Engaged.

    That's the point of this whole thread. That's why I can't do your
    experiment.
    Because you're asking the wrong questions for not having properly read
    the post?


    jaybee
     
    Jacques E. Bouchard, Aug 3, 2008
    #45
  6. Jacques E. Bouchard

    Larry in AZ Guest

    Waiving the right to remain silent, "Jacques E. Bouchard"
    Simple. The lens isn't as sharp at f3.7 as it is at f9.5, and/or the ND
    filter is introducing some distortion.

    The lens *IS* focusing, but not as well...
     
    Larry in AZ, Aug 3, 2008
    #46
  7. I reviewed all your previous posts and never got the
    impression that you COULD NOT focus with the ND
    filter in. The message was that your camera merely
    *changed focus" or *went out of focus* when you
    flipped the ND filter in.

    So with the ND filter in, how far away CAN you
    focus? (Still part of the differential diagnosis process)
    Are you saying that you can't focus at ANY distance
    with the ND filter in?

    If you look differentially (flipping back and forth) at
    your two example still images, you will note that not
    only is the ND example out of focus, but the frame
    also changes, as if you zoomed out. There appears
    to be more space between the satellite dish antenna
    and the right edge of the frame. And between the
    skylight and the right edge of the frame.

    This is NOT the way a proper ND filter should affect
    the image. Can't tell from your single data point whether
    this is a congenital design flaw, or a defect in your
    particular camera.

    There is also a significant difference in the green foliage
    in the background between the two images. Can we
    assume this was a result of the wind blowing in the trees,
    or is this another optical aberration?
     
    Richard Crowley, Aug 3, 2008
    #47
  8. You wrote "the DoF remains huge whatever the f-stop"
    The cited article says "decreasing the f-number decreases
    DOF".
    Your owns results appear to contradict that theory.
     
    Richard Crowley, Aug 3, 2008
    #48
  9. Jacques E. Bouchard

    Spex Guest

    Keep looking foolish.

    It's not me that's trying to save face. After reading later posts of
    yours it appears that you are now starting the moving target tactic.
    You never mentioned that you couldn't focus at all at f3.7 just the
    camera went out of focus when switching in ND filters. You now want to
    take the argument off in another direction to avoid your embarrassment.

    You continue to avoid pertinent questions regarding your test
    methodology too making your case weaker. Why won't you repeat the test
    first focusing up at F3.7?

    The images you've posted to flickr clearly show you are not focussed at
    infinity at either F9.5 or F3.7. The symbol next to MF i.e. 8- does not
    indicate infinity it indicates over infinity, that you are out of focus
    at the infinity end. My camera does it, in fact, all Canon XH-A1s and
    XL-H1s do it.

    The fact that the F9.5 image looks in focus is due to the aperture size
    and therefore the huge DOF as opposed to the narrower DOF at F3.7 which
    just shows how far your focus was before at F9.5.

    Until you can post images that prove otherwise, I conclude from your
    tests that they have exposed your sloppy camera work rather than a fault
    with the camera.

    It should be quite easy to show you camera _not_ focusing at F3.7 over a
    range of focus points.

    Have a wonderful day.
     
    Spex, Aug 3, 2008
    #49
  10. Jacques E. Bouchard

    Bill Fright Guest

    wow nice wadded panties.

    I understand out of focus at the far end of the lens. I'm just trying to
    get you to explain how you measure over infinity.

    Is this your first rodeo?
     
    Bill Fright, Aug 3, 2008
    #50
  11. Jacques E. Bouchard

    Bill Fright Guest


    I suspect you're right. Has this problem always been there or did it
    develop over time?
     
    Bill Fright, Aug 3, 2008
    #51
  12. Actually it would be better to focus on f/3.7, because your focus can't be
    fooled by the larger DoF of the f/9.5.

    cheers

    -martin-
     
    Martin Heffels, Aug 3, 2008
    #52
  13. Your kidding yourself, right? :)

    -m-
     
    Martin Heffels, Aug 3, 2008
    #53
  14. "Martin Heffels" wrote ...
    Lower f-stop number = shallower DOF, regardless
    of magnification, imager, or any other factor.
     
    Richard Crowley, Aug 3, 2008
    #54
  15. Jacques E. Bouchard

    Spex Guest

    Nobody is measuring over infinity you know damn well that phrase is not
    meant to be taken literally. You are just trying to score a cheap shot.
    The camera's firmware knows when the focus element has moved too far
    to be focussed at infinity. What's so difficult to understand?

    The camera costs a tenth of a good Broadcast HD lens so compromises are
    made. A small price to pay in my view.

    You can set up a focus preset to infinity, if one so desires, that can
    accessed at a touch of a button if you need infinity regularly.
     
    Spex, Aug 3, 2008
    #55
  16. Martin Heffels, Aug 3, 2008
    #56
  17. Jacques E. Bouchard

    Spex Guest

    Spex, Aug 3, 2008
    #57
  18. Richard Crowley, Aug 3, 2008
    #58
  19. Jacques E. Bouchard

    Toby Guest

    You could also try the same experiment with one addition: crank up the gain
    with the ND filter in place so that you are back down to a smaller aperture
    and check the result. Of course the image will be more noisy, but if you do
    not see the same degradation it will indicate a problem with the lens. If
    the problem remains it indicates a problem with the ND filter.

    Toby
     
    Toby, Aug 6, 2008
    #59
  20. I just got tired of playing your childish name-calling game, Spex. Back up
    your "professional" opinion with a real name some time and we'll talk about
    giving you credibility.


    jaybee
     
    Jacques E. Bouchard, Aug 25, 2008
    #60
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