Internal ND filters on the XH-A1 affecting focus

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

  1. I guess Canon skimped on their internal ND filters then...


    jaybee
     
    Jacques E. Bouchard, Jul 30, 2008
    #1
    1. Advertisements

  2. I noticed today that using one or both of the Canon XH-A1's internal ND
    filters considerably affects the focus - which, when already focused at
    infinity, leaves me with a blurry picture I can't adjust.

    Is this a phenomenon common to ND filters, or just the internal ones on the
    XH-A1?


    jaybee
     
    Jacques E. Bouchard, Jul 31, 2008
    #2
    1. Advertisements

  3. Okay, I just took footage of a distant rooftop outside my office window by
    replicating the exact conditions (tripod-mounted, full zoom in, focus set
    to infinity). The camera settings are clearly displayed on each picture.
    Look at the brick on the chimney.

    Tell me what the operator error could be. I certainly hope it's something
    silly that I did, because I don't like what I see.

    http://www.flickr.com/photos/[email protected]/


    jaybee
     
    Jacques E. Bouchard, Jul 31, 2008
    #3
  4. A-ha! Martin, you're a prince. Now, why did I only notice this when
    zoomed in?

    http://www.flickr.com/photos/[email protected]/

    Thanks for your knowledgeable input.



    jaybee
     
    Jacques E. Bouchard, Jul 31, 2008
    #4
  5. This is not normal - ND filters normally have no diopter power...
    --DR
     
    David Ruether, Jul 31, 2008
    #5
  6. Jacques E. Bouchard

    Spex Guest

    I suspect that the aperture is set to automatic and as the ND filters
    are selected the aperture opens and the DOF reduces. The once in focus
    subject would then go out of focus.

    I have an XH-A1 and have no problems at all. It is a wonderful camera.
     
    Spex, Jul 31, 2008
    #6
  7. Jacques E. Bouchard

    Spex Guest

    Actually I suspect your problems are operator error not the quality of
    the XH-A1 components.
     
    Spex, Jul 31, 2008
    #7
  8. The camera was tripod-mounted, in full manual mode, zoomed in at the
    maximum on an object about 1 km away, with focus set at infinity. When I
    switched on the ND filter, the far away object that was in focus became
    blurry.
    Try replicating the above conditions.


    jaybee
     
    Jacques E. Bouchard, Jul 31, 2008
    #8
  9. Internal filters actually do indeed affect the back-focus of the lens,
    because they increase the length of the optical path. This will usually
    only be visible on wide angles, and when trying to focus on infinity, turns
    out your image gets blurry. So either zoom in a bit, or use external
    ND-filters or a higher shutter-speed.

    Cheers

    -martin-
     
    Martin Heffels, Jul 31, 2008
    #9
  10. ?????????????
    Unless the lens optics are moved to accommodate the filter thickness
    (highly unlikely), there should be no effect on the optical performance
    of the lens regardless of where it is placed (assuming high quality
    filters). You may see a *slight* shift in focus with the addition of a
    rear filter on a lens, though. Putting in place a ND filter with a fixed
    shutter speed would force the aperture wider for the same exposure
    under the same conditions (as would using a higher shutter speed). A
    lens witth inherent problems would then be more likely to show what
    JB has observed. Zooming in toward tele would also likely worsen
    the problem seen here, since to me, it looks like it is caused by a
    defective lens. It is hard to tell, but it looks like in the image without
    the filters that the sharpness in the left 1/3rd or so isn't great, but the
    center and right are OK. With the ND filters in place, the image is
    kinda OK in the bottom-right (with only the chimney on the right OK),
    but quite soft elsewhere. I would expect better performance (my
    HV20 lens is quite sharp to the corners wide open in the first half
    of the VF-marked zoom range, but it gets rather poorer toward the
    corners as the long end is approached (but the decline is even around
    the frame edges and corners). I would do further checking on the
    lens, without the filters in place...
    --David Ruether
    www.donferrario.com/ruether
     
    David Ruether, Jul 31, 2008
    #10
  11. Jacques E. Bouchard

    Larry in AZ Guest

    Waiving the right to remain silent, "David Ruether"
    In the "no filter" position, there should actually be a piece of clear
    glass, so that the optical path is identical to that of the filters.

    Otherwise, there will be a shift in back-focus between no filter and
    filter positions.

    Especially with these small format cameras. The smaller the imaging
    device, the more critical this becomes. It's much less noticable on 2/3"
    and larger format cameras.
     
    Larry in AZ, Jul 31, 2008
    #11
  12. I'd be curious to see a test (under the same conditions) done by another
    XH-A1 owner. It's not critical if this proves to be a hardware
    limitation, but I just want to make sure there's nothing wrong with my
    (nearly) brand-new camera.


    jaybee
     
    Jacques E. Bouchard, Jul 31, 2008
    #12
  13. ALWAYS check out new gear (never make the assumption that
    'cuz it's new, it's good...!). It is reasonable to expect that the lens
    will be reasonably sharp to the corners wide open at all focal lengths
    (and it may be better than this - the VX2000 was, and the HV20
    is quite good short the longest tele range, where it is less good but
    not terrible). Find a location with a lot of fine detail at the same
    distance (distant cityscape, woods, even a brick wall if you are careful
    with alignment), shoot wide open at several different FLs, and shoot
    at least two different views with some subjects without changing
    anything but tilt (in order to have the same information in the top and
    bottom halves of the frame, if that is needed with the chosen subject).
    Check on a *good* 1080p HD TV (not much else will tell you much
    unless the faults are severe). Any slight faults should be symmetrical
    in the frame (i. e., all four corners equally *slightly* soft at worst -
    otherwise I would try to get a replacement). And, I think "Larry" is
    right - you should see no difference in focus with the ND in or out
    with a clear glass substitute likely used in the lens. BTW, "wandering
    focus" within a video image still frame is a "give-away" that something
    is likely wrong with the lens...
    Good luck with this!
    --David Ruether
    www.donferrario.com/ruether
     
    David Ruether, Aug 1, 2008
    #13
  14. In theory. But "should" and "likely" aren't quite enough for me to pack
    this thing, ship it at my expense to the service centre and wait 4-6
    weeks to get it back - which is why I'd like to see someone else with an
    XH-A1 do the same test and see what they get.
    I checked the original high-res captures after reading your comments. I
    couldn't see any difference in focus in the areas you mentioned.


    jaybee
     
    Jacques E. Bouchard, Aug 1, 2008
    #14
  15. Jacques E. Bouchard

    Greg Melton Guest


    Just because a lens is set to infinity doesn't mean it is focused even
    if it is beyond 50ft or 100ft. I'm in the same camp as Spex. It's a
    DOF issue with your aperture. Even on a 2/3" lens, you'll have the
    same phenomenon. Trusting your eye in manual focus is the best
    defense.
     
    Greg Melton, Aug 1, 2008
    #15
  16. It was beyond 3,000 feet. As I said, it was focused at an object 1 km
    away, and became out of focus when I engaged the ND filter. I provided a
    link to pictures to illustrate the effect.

    It's really quite simple, there is no DOF issue here, not at 3,000 feet
    away. Do take the time to look at the pictures, they speak for
    themselves.

    At this point, I'm discounting all speculations until someone replicates
    the experiment and arrives at a different result. I'm hoping someone will
    kindly volunteer, there seems to be plenty of XH-A1 owners here.


    jaybee
     
    Jacques E. Bouchard, Aug 1, 2008
    #16
  17. Jacques E. Bouchard

    Spex Guest

    You weren't even focussed up at infinity, 8- is not infinity focus.

    It is absolutely crazy to expect there not to be some effect from the
    DOF when the aperture changes from F9.5 (an enormous DOF) to F3.7 ( a
    much reduced DOF). I mean it is almost as absurd as focusing up on a
    subject at a wide angle then zooming in and expecting the subject to
    still be in focus. You just don't do it.

    No camera operator worth their salt would ever expect the subject to
    remain in focus with such as drastic change in aperture.

    You don't need other people to repeat your flawed experiment. Instead
    repeat it yourself. This time focus up with the NDs engaged and the
    aperture at 3.7, then disengage the NDs. If this second image goes out
    of focus then you've got a duff camera if it remains in focus** you were
    wrong all along.

    **Above F5.6 you get a small amount of softening due to lens diffusion
    which you should see. This is to be expected and is not a fault and
    nothing to do with the back focus.
     
    Spex, Aug 1, 2008
    #17
  18. It's as far as the lens will focus. When ND is set, I can't focus anymore
    on a subject that's at 3,000 feet.
    Focusing on a subject 3,000 feet away: at 9.5f the near limit is 403
    feet; at 3.7f the near limit is 878 feet. In both cases, the far limit is
    infinite. That means that when you're focused on a subject 3,000 feet
    away, you can set the aperture to any damn setting you want and your
    subject is going to stay in focus.

    http://www.dofmaster.com/dofjs.html
    None of the DOF theories make sense. Not at 3,000 feet. I've just
    demonstrated this.
    The experiment is not flawed, it just *IS*. The images were not doctored,
    they're not biased and they don't have an agenda. They show a clear
    picture well beyond the near limit for its f-stop, and the same image
    blurred by using the ND filter, again beyond the near limit for its f-
    stop.

    I'm just trying to explain what the pictures showed. It's not a matter of
    personal interpretation, the evidence is clear for everyone to see. And
    DOF is not a factor here, not at 3,000 feet, not at either f-stop
    settings.

    And I wish you'd refrain from trying to make this personal. Failing that,
    please feel free to refrain from this otherwise civil and reasonable
    discussion.



    jaybee
     
    Jacques E. Bouchard, Aug 1, 2008
    #18
  19. "Spex" wrote ...
    If the back-focus is adjusted properly, an object will remain
    in focus through the entire zoom range.

    Of course small cameras with built-in lenses don't necessarily
    make it easy (or even possible?) to adjust back-focus. OTOH,
    back-focus is a common maintenance adjustment on larger
    cameras with interchangeable lenses.

    The concept of back-focus does not necessarily have
    anything to do with the presenting symptoms of this
    discussion.
     
    Richard Crowley, Aug 1, 2008
    #19
  20. Jacques E. Bouchard

    Spex Guest


    Richard this has got eff all to do with back focus. It's all to do with
    craftsmanship.
     
    Spex, Aug 1, 2008
    #20
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.