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Thread: Correcting Focus When Stacking Filters

  1. #11
    Nodda Duma's Avatar
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    Re: Correcting Focus When Stacking Filters

    Jim Noel is spot on. Theoretically, filters should contribute no optical power and therefore would not affect the image. In the real world, consumer-grade filters do have an impact... not focus per se, but degradation of the wavefront and all sorts of other effects.

    If you want to be able to do what you want to do, buy filters that you can review the specs for, and ensure that the tolerances on wedge, wavefront error, and surface flatness are all minimized. If the filter you have/are looking at doesn’t specify these, then assume it’s not a good enough filter. Otherwise, you’ll need to figure out how to focus with them on to at least minimize that contribution.
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  2. #12

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    Re: Correcting Focus When Stacking Filters

    The Cinema folks stack multi filters all the time. ND filters being most common with color correction (film), soft matt or similar and ... They all go into slots at the back of a Matt Box specifically designed for this.. Most common problem with filter stacking is increased flare, reduced light transmission and all related to disrupting the optical path. In theory, if the filters are optically perfect, there is no difference, in reality NO filter is optically perfect and real world trade offs must be made when using any filter (s) in front of the lens or in the back of the lens.

    With this said.

    Why 12-2/3 stops in an attempt to produce glow from moving water? What was the exposure time?

    It is possible the out of focus could be due to movement in the camera, subject or ? due to the ? exposure time.

    ND filtering down 12-2/3 stops is not needed to achieve glowing water effect from moving water.

    Suggestion, don't rely on filters alone to achieve an expressive image.



    Bernice



    Quote Originally Posted by AdamD View Post

    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.

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

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  3. #13

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    Re: Correcting Focus When Stacking Filters

    I agree with the others that a focus shift do to stacking filters is "theoretically" not your problem, however, stacking so many filters introduces a lot of other possible causes for image-degradation.

    The first that comes to mind for me is the extra-long exposure you're going to have. Sure, you wanted to blur the waterfall, but you likely also blurred the mourning dove as it probably wasn't sitting absolutely still for the entire exposure.

    Long exposures also show more degradation from camera movement and negative pop/movement.

    The accumulated imperfections in each of the filters, which wouldn't degrade an image appreciably when used alone, add up. If you make a practice of stacking filters, invest in the best filters your money can buy.

    Lots of uncoated surfaces in front of the lens = lots of flare. That can reduce contrast appreciably, especially in the least-dense areas of the negative.

    And, lastly, fiddling around mounting all those filters after focusing is almost certain to shift focus a bit. Too bad you can't check focus after mounting the filters. Maybe add them all but the strong ND filter, check focus, and then carefully add the ND filter?

    Best,

    Doremus

  4. #14
    Drew Wiley
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    Re: Correcting Focus When Stacking Filters

    More air/glass interfaces, and not sealed ones, but isolated surfaces greatly more subject to haze, dust accumulation, humidity fogging, physical damage, etc. It all adds up. People working on movies sets were expected to be real pros, but they certainly weren't shooting large format frames either! I have chatted with a few, and it amazed me how much they really knew about film, lenses, and metering. But any momentarily individual projected cinema frame is not intended to be examined up close or pontificated about like our large format framed prints. Just the sheer cost of LF film these days, esp color, would seem incentive enough to keep the light path as clean as possible. But if someone happens to like the look of vaseline over the lens, or aphids crawling between filters, it's not illegal.

    But if water can't be distinctly blurred by a 1 or 2 sec exp, it sounds more like a swamp than a waterfall. Sometimes I've wanted to slow down an exposure dramatically in relation to a particular lens without shutter, where I use a simple lenscaps exposure method instead, and might additionally want grass blurred by the wind while things like rocks and textured tree bark remain totally crisp. A small f-stop, med speed film, and basic 47 deep blue or 29 deep red contrast filter will easily slow things down enough for that kind of project.

  5. #15
    Alan Klein's Avatar
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    Re: Correcting Focus When Stacking Filters

    Unless I missed it, you never mentioned what aperture you used. What was it? Where did you focus? What was the DOF?

  6. #16

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    Re: Correcting Focus When Stacking Filters

    Quote Originally Posted by Alan Klein View Post
    Unless I missed it, you never mentioned what aperture you used. What was it? Where did you focus? What was the DOF?
    Alan, you didn’t miss anything...
    I shot this at f/16 for 8 seconds in full on Arizona sun. I focused on the rock waterfall.

    This was more of an experiment to see if I could use a bunch of filters to get the shutter slow enough to blur the water.

    I’m gathering from your collective answers that the focus error was more likely user error as opposed to filter stacking error. Which is a good thing. I can say this, I probably should have added a stop more light or maybe 2/3 more instead of the calculated value.

    As always, I would never have picked up a LF camera had it not been for this forum!! So thank you!
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  7. #17
    John Olsen
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    Re: Correcting Focus When Stacking Filters

    Quote Originally Posted by Jim Noel View Post
    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.
    I agree with Jim. The few mm's of filter can't have any effect on your focus as long as they're in front of your lens. It's just too many surfaces and too many chances for slight uncleanness to build up fuzziness. Now, if they were behind your lens, then every mm of glass would require 1/2 mm of focal correction.

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