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Thread: Correcting Focus When Stacking Filters

  1. #1

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    Correcting Focus When Stacking Filters

    Hi all, here's my next installment of technique questions. This one is about correcting focus, when you stack multiple filters onto a lens.

    Ok. I'm going to post a photo (see below). You'll immediately see that it's not exposed correctly, but I want you to look past that and zoom in a bit. You'll see the focus is off. The waterfall rocks are very soft and notice the morning dove on the fence, you can barely tell it's a dove!!

    When I had this image focused, everything looked perfectly sharp, even the dove! AFTER I shot it, everything was sharp (except the dove, she flew away). But when I developed it, you can see, it's not very good. I checked the negative and it matches the image here. So something went wrong.

    I believe my problem was filter stacking. In this case, I wanted to get a little wild. I used the following: Red, ND2, CP and a 6-stop ND (100x100). That's 12-2/3 stops in all. I was trying to get the waterfall to blur but you can't see that because I didn't get the exposure right.

    I did a little digging and asked a few questions and I believe my problem is that with all those filters, I moved the focus point off the film plane.

    So, if that's correct, how the heck are you supposed to refocus after stacking all those filters? I can't even see the image on the GG. My understanding is also that you should focus using stacked UV filters to achieve the same focal distance, but I don't own any UV filters for my LF lens, let alone 3 of them.

    When technique do you use to ensure proper focus when stacking filters?

    Click image for larger version. 

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  2. #2

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    Re: Correcting Focus When Stacking Filters

    And you can see here where I got it right. But in this case I waited for better light and stacked fewer filters. This was had a Red and 6-stop ND (that square filter).

    Click image for larger version. 

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  3. #3
    Drew Wiley
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    Re: Correcting Focus When Stacking Filters

    I never stack filters; that helps. Otherwise, use a good loupe and practice with something appropriately lit till you can actually see the effect in the groundglass. A piece of reflective aluminum tinsel out in the scene hit by a laser pointer or other bright light might be a good place to start.

  4. #4

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    Re: Correcting Focus When Stacking Filters

    I can’t tell from the posted images if the whole thing is soft or if it is out of focus.

    Someone can correct me if I’m wrong but stacking filters on the front of the lens should not require refocusing. You only need to focus with a filter in place when it is behind the lens.

    Maybe stacking all those filters simply degraded sharpness because one or more of them is crap. If they aren’t coated, stacking filters can also add flare which reduces contrast.

  5. #5

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    Re: Correcting Focus When Stacking Filters

    Those are interesting ideas.

    I can say this, individually each filter is good. I also noticed that the uploaded images are hard to tell of it's soft or out of focus like Michael R was saying. I can tell looking at it, it looks way too soft and must be out of focus to me....

    Thanks for the tip!
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  6. #6
    ic-racer's Avatar
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    Re: Correcting Focus When Stacking Filters

    Try focusing with the filters in place.

  7. #7

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    Re: Correcting Focus When Stacking Filters

    IC-RACER, that's exactly what I can't do. This light is too dark after stacking all those filters to get the shot.
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  8. #8

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    Re: Correcting Focus When Stacking Filters

    This doesn’t make sense other than the filter stack degrading the quality due to its impact to the MTF (aberrations, etc.) because they’re not “perfect”. Otherwise, if you placed a perfect glass in front of a lens focused to infinity, there would be absolutely no effect on imaging aside from some stray light considerations.

    Often times having a resin filter (grad filter) can cause some very slight degradation, often not obvious at all, and so do other filter types. By themselves you most likely won’t be able to tell unless you have an MTF bench. But if you stack and stack you can add up to a substantial amount of aberrations.

    The experiment would be to take multiple shots of a flat target (eg USAF 1951 chart or equivalent) as you add one more filter at a time, then one again at the end without them to make sure nothing has moved. That’ll give you an empirical limit of how and what filters you can combine.

  9. #9

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    Re: Correcting Focus When Stacking Filters

    If your filters were each optically flat, and absolutely clean, they should make no difference in focus. BUT unless you paid a rather high amount for each filter i doubt that any of them were absolutely optically flat, or that they were meticulously clean. Therefore fuzzy, indistinct image.
    If I use ND filters, they are single gels so there is no optical interference and are behind the lens eliminating flare.
    These are leftover habits from when I thought every image had to be very sharp. Since I have decided that images look better if they see as the eye sees this ultra sharpness is not important to me, except perhaps in one tiny area.

  10. #10
    ic-racer's Avatar
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    Re: Correcting Focus When Stacking Filters

    Quote Originally Posted by AdamD View Post
    IC-RACER, that's exactly what I can't do. This light is too dark after stacking all those filters to get the shot.
    Maybe more time under the dark cloth to adjust your eyes.

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