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Thread: Using Center ND filters on the back of LF lenses a viable option?

  1. #1

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    Using Center ND filters on the back of LF lenses a viable option?

    Has anyone tried this?

    This is my predicament: a Schneider 95mm Schneider center ND filter (IVa-10591 with 1.5 exposure correction) for my 90mm Super Angulon XL seems impossible to find on the used market. New ones are special order and go for over a grand. Now the rear of the 90mm Super Angulon XL is not threaded but would easily allow one to slip in an 82mm Center ND filter: Schneider IV, Heliopan ND, or Rodenstock E82/112 (all 1.5 exposure correction). Slipping in a thin rimed 82mm filter into the back of the XL seems to cause a little vignetting, but not enough to really matter, this I have to check out further. Cost wise, have seen used 82mm center ND filters go for 1/3 of the price of a 95mm Schneider.

  2. #2

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    Re: Using Center ND filters on the back of LF lenses a viable option?

    No, it belongs in front, and in front it won’t cause a focus shift. Plus, the falloff when the light enters the lens, it has already occurred after it leaves the lens.

  3. #3
    C. D. Keth's Avatar
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    Using Center ND filters on the back of LF lenses a viable option?

    Viable is kind of the key word here. Could you do it? Yes. The trouble is that I don’t know how you would know what filter gives the right size area of ND and the right amount of softness to that spot since all the center filters are intended for the front. You’d be stuck with figuring it out by trial and error.

    Also, a filter on the rear will require you to compose and focus with the filter in place because it will cause a focus shift. I know I wouldn’t be crazy about composing a stop and a half darker than I have to.


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    Re: Using Center ND filters on the back of LF lenses a viable option?

    Quote Originally Posted by Bob Salomon View Post
    No, it belongs in front, and in front it won’t cause a focus shift. Plus, the falloff when the light enters the lens, it has already occurred after it leaves the lens.
    Hum--this got me thinking. I agree that glass filters should be placed in front of the lens because of the focus shift but does it really matter if you are focusing with the filter in place? As far as the falloff occuring after it leaves the lens, really? Back in the day when I was doing a lot of black and white printing I would often burn in the corners of the print--partially to bring attention to the subject and partially to compensate for the light falloff, especially when using a condenser head enlarger. In any case, you place your fist between the lens and the paper because of course you can't put your fist between the light source and the negative, right?

    I don't have any experience with center ND filters so take my next comments with a grain of salt.

    The light falloff changes depending on what f/stop you are using so why is there only one center ND filter per lens, if you're going for OCD precision shouldn't there be one for each f/stop? Well, maybe not every f/stop but you catch my drift.

    Have you considered making your own filter? Clear acetate or polyester sheets are rather easy to find, many art supply stores sell them, and you can also get various inks and dyes that can be used in an airbrush. Another option is to cut a hole in a piece of black paper, put it on a light box and take a picture of it out of focus. Then use the black and white negative like you would a gel filter. Yeah I know, base plus fog, not ideal, blah blah, but it should be possible to come up with something that works good enough without having to spend your kid's inheritance on a center ND filter.

  5. #5

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    Re: Using Center ND filters on the back of LF lenses a viable option?

    Quote Originally Posted by dfort;1535793I don't have any experience with center ND filters so take my next comments with a grain of salt.

    The light falloff changes depending on what f/stop you are using so why is there only one center ND filter per lens, if you're going for OCD precision shouldn't there be one for each f/stop? Well, maybe not [I
    every[/I] f/stop but you catch my drift.
    Only a grain?

    "Falloff" has two causes. You're conflating them.

    Mechanical vignetting, as when the rear cell's barrel cuts off the outer part of the cone of rays the lens projects. Stopping down makes the lens' exit pupil smaller, reduces this effect. That's why manufacturers of CFs as the OP wants strongly recommend stopping down at least two stops when using one. Otherwise mechanical vignetting will dominate and the CF won't help.

    Optical vignetting, good ol' cos^4. This isn't sensitive to aperture.

    I have 4 CFs, the most I paid was around $200 plus postage. There are bargains around. If you don't believe me, look at sold CFs on ebay. Real bargains aren't common, one has to be patient and look, um, creatively. The OP is a bit more stuck than most because he needs an uncommon size.

    Greg, if I were as stuck as you I'd think hard about getting an 82 mm CF and trying it out. You should be able to resell it if it doesn't work out for not too large a loss ...

  6. #6

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    Re: Using Center ND filters on the back of LF lenses a viable option?

    So earlier today mounted my 120mm f/8 WA Nikkor on my 4x5 Sinar X. Carefully focused the camera without the center ND filter and moved the back standard till it fell on the center "0" on the measurement scale on the bottom of the rear standard. Put on the filter and refocused - ended up essentially on the same "0" point. Removed the lens and temporarily attached the 77mm filter to the back of the lens with Gaffer's tape. Refocused again and again ended up on the same "0" point on the scale. Obviously there is a shift in focus when the filter is placed in the optical path, but not enough to practically matter, especially since I always use the 120mm Nikkor WA stopped down.

    This week will shoot two identical shots with the filter in front of and in back of the lens and see if there is any difference. If not or very little, then per Dan Fromm's post: "Greg, if I were as stuck as you I'd think hard about getting an 82 mm CF and trying it out. You should be able to resell it if it doesn't work out for not too large a loss ..." will be on the lookout for an 82mm Center ND filter: Schneider IV, Heliopan ND, or Rodenstock E82/112. Any of the three in used condition has to be less than a new 95mm Schneider center ND filter.

    Will post how things go.

  7. #7
    C. D. Keth's Avatar
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    Re: Using Center ND filters on the back of LF lenses a viable option?

    The focus shift of a filter behind the lens will be 1/3 the thickness of the filter. It’s small but it’s there.


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  8. #8

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    Re: Using Center ND filters on the back of LF lenses a viable option?

    Quote Originally Posted by C. D. Keth View Post
    The focus shift of a filter behind the lens will be 1/3 the thickness of the filter. It’s small but it’s there.


    Sent from my iPhone using Tapatalk
    Thanks. Per my caliper's measurement, my filter is about 3mm thick... a 1mm focus shift I can easily deal with especially since I always use the lens stopped down.

    I have a 500mm Mirror Nikkor lens that takes a 39mm thin filter screwed into the rear of the optic. Nikon states that if the filter is removed, it will effect the sharpness of the image. Once I made several exposures with and without the filter in place, and focused with an eyepiece magnifier. The exposures made without the filter in place were almost imperceivable less sharp that the exposures made with the filter in place, but they were definitely not as sharp. Nikon always claimed that the filter was part of the optical design of the lens.

  9. #9
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    Re: Using Center ND filters on the back of LF lenses a viable option?

    Focus shift will have a far greater effect on a 90mm wide angle lens on 4x5 than on a 500mm lens on 35mm film -- sad to say.
    Last edited by rfesk; 2-Feb-2020 at 19:14.

  10. #10
    C. D. Keth's Avatar
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    Re: Using Center ND filters on the back of LF lenses a viable option?

    If that is the way you want to go, you can compose with a clear filter of the same thickness to induce that focus shift. Replace it with the taking filter after you compose and your focus will be spot on.


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