Focus Issue when using 2-Stop ND Filter

Discussion in 'Digital SLR' started by CJS, Mar 1, 2007.

  1. CJS

    CJS Guest

    Hi all,

    I'm puzzled by this, and I'm hoping that someone can shed some light on it
    for me.

    I've just shot about 60 photos of my daughter with a 20D - 70-200 F2.8L IS
    combination, complete with a 2 stop ND filter (trying to capture the remains
    of a sunset in the background whilst foreground is illuminated by 2 remote
    (and diffused 580s).

    confirmation each and every time (camera set to 1 shot autofocus, neither me
    or subject moving), AF set to centre (mid torso), and later on upper AP
    point (face)

    I was shooting at 2.8 @ about 1/90th @ about 70mm @ 100 ISO, with IS on.

    Back in "the lab" (ok, my recliner chair) I tried to isolate the problem -
    and it appears to be the 2-Stop ND filter (it's a cheap brand).

    To eliminate hand shake I mounted the camera on my trusty Gitzo 1548 - I
    enabled mirror lockup & timer - and did several shots with and without the
    ND filter - every time the ND filter is on I get massive "out of focus" -
    kind of - what I'm getting looks EXACTLY like top-left to bottom-right
    camera shake. Thinking it might be that the autofocus is struggling in the
    low light I took off the ND filter - auto-focused - switched the lens to
    manual focus - took a shot - put the ND on took another shot - took it off
    again - took a 3rd shot. Result? Before and after tack sharp, with the ND
    attached, blurry.

    I've examined the filter and it looks normal. Only other anomaly is I've
    done all this without a lens hood (left it at work).

    This has got me stumped. Any ideas anyone?
    CJS, Mar 1, 2007
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  2. I have used the same combination as you (camera, lense and ND filters). My
    ND filters are Hoya filters ND4 and ND8. I have never had the problem you

    Perhaps your filter is not good enough and it disturbe the auto-focus
    Jørn Dahl-Stamnes, Mar 1, 2007
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  3. CJS

    CJS Guest

    That's what I wondered, but I've still got the problem with the lens set to
    manual focus.
    CJS, Mar 1, 2007
  4. Canon 20D AF works up to about f5.6. F2.8 + 2 stop ND = f5.6.
    Therefore AF may not work.. or in your case, doesn't.

    So avoid the ND, and either use higher shutter speeds, shoot at a
    different hour, set your flashes up differently...
    mark.thomas.7, Mar 1, 2007
  5. CJS

    Matt Clara Guest

    You're confusing your terms. While a 2 stop filter will lower the amount of
    light entering your camera, it doesn't make it f5.6--that's a ratio defined
    as the focal length of the lens divided by the effective diameter of the
    aperture. A filter doesn't change that. If you mean the 20D AF doesn't
    work well in low light, that's something else altogether.
    Matt Clara, Mar 1, 2007
  6. Quite correct, had not put brain in gear.

    But I think my recommendations are still correct, and clearly the
    filter is 'somehow' causing the af to go haywire, so low-light/lack of
    contrast in subject in the AF zone is all that is left.

    I remain a bit dubious about the concept of using an ND in his
    mark.thomas.7, Mar 1, 2007
  7. CJS

    Lionel Guest

    Given that it's a cheap filter, it might be something as simple as
    less than stellar glass in the filter, or cloudiness in the actual
    dark layer, causing it to soften your photos. Bear in mind that at two
    stops down, it'll likely be harder for you spot a softish image
    through the viewfinder.
    I'd test that hypothesis by taking the filter off the camera & holding
    it in front of a light bulb,to see if things look sharp through it to
    my own eyes.
    Lionel, Mar 1, 2007
  8. Agree, I have used a ND4 in combination with a ND8 on a 70-200 2.8L IS and
    the auto-focus still worked.
    Jørn Dahl-Stamnes, Mar 1, 2007
  9. CJS

    CJS Guest

    Thanks for the reply, but I can't see how it's an AF issue when I get the
    same results with the AF switched off. As I said originally I took the ND
    off - auto focused - switched the lens to MF - took a shot (to check lens
    still in focus; it was ) - put ND on - took another shot (poor focus) - took
    ND off - took third shot (to check I hadn't moved focus ring; shot in
    CJS, Mar 1, 2007
  10. CJS

    frederick Guest

    So do I - it seems totally pointless.
    OTOH a grad ND filter may be useful.
    frederick, Mar 1, 2007
  11. I find it hard to comprehend that even a *cheap* filter could do this
    - can you post the images, or crops thereof?
    mark.thomas.7, Mar 1, 2007
  12. CJS

    Matt Clara Guest

    I once tried to shoot a hawk out my kitchen window, which had old glass, the
    kind with mild distortion. Still you could see the hawk clearly through it.
    No matter how I tried, I could not get a good focus on him, until I removed
    the old Kenko UV filter from the lens, which was a 1960's Pentax 300mm f6.8
    (or some such). Filters do make a difference, however so slightly, cheap
    ones likely more than good ones.
    Matt Clara, Mar 1, 2007
  13. Somewhere I have a tiffen circular polarizing filter
    that had this same problem (back in film days of the
    early 1990s). The filter simply blurred the images.
    I took a micrometer to it and measured the thickness
    of the glass near the edge and found it had a measurable
    wedge. So, it does happen.

    Roger N. Clark (change username to rnclark), Mar 2, 2007
  14. CJS

    CJS Guest

    I'm deliberately shooting F2.8 with the 70-200 F2.8L IS because I need a
    very shallow DOF. At F2.8 I need a shutter speed of around 1/800th to
    1/1000th. Setting sun is behind subject, so need lots of fill flash. It is
    my desire to not use high-speed sync, so need to keep shutter speed <
    1/250th. 2 stop ND drops shutter speed to around 1/200th.

    All a grad ND would do help brighten the standing subject from about the
    knees down (setting sun and subject are in the top 3/4 of the shot).

    Did I miss anything?
    CJS, Mar 2, 2007
  15. CJS

    CJS Guest

    I found it hard to comprehend too, but it's the conclusion I was forced to
    come to. I was shooting almost exactly the same setup on several occasions
    the last week, except no ND filter - high shutter speeds, and HSS on the
    580s, and almost every shot was tack sharp.

    I could almost explain it away with camera shake ... until I got exactly the
    same results with the camera on a Gitzo 1548 and both timer and mirror
    lockup enabled (if you don't know that particular tripod it's one you could
    probably prop a corner of a house up with while you changed the piles!)
    Hell, I even tried shooting on the tripod and changing the ISO so that the
    shutterspeed stayed the same - same result. It's almost as if the image was
    bouncing around between the ND and the front element of the 70-200 a few

    Unfortunately I can't put any images up at the moment (I killed my pbase
    account due to lack of use), but I have discovered an effective 2-step work

    "I took 2 steps and threw both cheap/nasty ND's into the bin and then
    ordered a singh-ray variable ND filter from their site".

    If it turns out to be some weird 20D issue I'm not particularly worried
    either as I ordered one of the new 1D Mk.3's :) (Just had to mention that :)
    CJS, Mar 2, 2007
  16. CJS

    frederick Guest

    You said you were shooting at 1/90th.
    frederick, Mar 2, 2007
  17. CJS

    CJS Guest

    I said "about 1/90th". It started out about 1/200 and ended up at 1/90th as
    the sun dropped further. But even at 1/90th if I drop the 2-Stop ND I end up
    at "1/360th" - still well above the X-Sync speed of a 20D.

    Without the ND the only other choice was to stop down - which doesn't look
    good on a 22" x 33" canvas when the background is tall weeds :(
    CJS, Mar 2, 2007
  18. Which should be relatively easy to fix in PS, no, using a layer mask and
    some blur
    Ed Ruf (REPLY to E-MAIL IN SIG!), Mar 2, 2007
  19. FWIW, I don't believe you ever mentioned before you where stacking cheap
    ND's. That kind of detail would have been valuable in the early parts of
    this thread.
    Ed Ruf (REPLY to E-MAIL IN SIG!), Mar 2, 2007
  20. CJS

    CJS Guest

    I wasn't stacking anything - only ever had the 2-Stop ND on the camera (also
    said in an earlier part of this thread that a grad ND doesn't work for this
    composition). I owned (note: "past tense") a grad ND of the same brand /
    "quality" (ie: apparantly cheap and nasty) - so both went in the bin.

    Old story I guess - you get what you pay for.
    CJS, Mar 3, 2007
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