Experimenting with stacked polarisers

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A rather interesting effect, for daylight. Note texture of water.

Some time ago, I went out hunting for a variable ND filter in anticipation of a beach trip. (I know, most people buy a swimsuit or sunglasses, but I’m a photographer; sue me.) What I found was a little troubling. Not only were the locally available options hideously expensive, but they also weren’t multicoated – this brings about an obvious set of flare related problems given the environments (sunny) under which they’d be used. I thought I’d get creative instead.

The var NDs work according to very simple principles of crossed polarizers cancelling each other out. I don’t know if they use linear or circular polarizers; (I suspect the latter, because the former interferes with the AF systems of modern cameras) however a little experimentation showed that a pair of circular polarizers would produce exactly the same effect. So, I walked out of the store with another Zeiss T* polarizer to stack on the one I’ve already got. Turns out you need one more piece to complete the puzzle: the polarizing element is mounted in the same (rotating, obviously) portion of the frame on both, and they have to be reverse-mounted because they come installed in the same orientation from the factory. So, after a little ebay hunting, a male-to-male reverser ring was on its way to me. I’m using a 62mm system, for several reasons – one, all of the filter threads on my Hasselblad lenses (the system I’m most likely to use the var NDs with) are 62mm; coincidentally, most of my larger Olympus M4/3 lenses are, too. Also, if you’ve looked at the cost of an 82mm Zeiss polarizer lately, you’ll know the other reason.

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Vignetting occurs at 24mm equivalent on the Olympus 12-40. But you can’t use a polarizer with this FOV anyway; it’ll result in the odd dark bands you see here.

All set. Use of the system – a polarizer, the ring, and another polarizer – requires some care; if you tighten things too much, you won’t be able to get them off again without cutting your hands on the filter threads. Too loose and obviously you’ll land up dropping something, probably one of the polarizers, and knowing Mr. Murphy, probably in a place where you’ll never be able to get at it again – but somewhere you can see it so it taunts your for you clumsiness. (I’m pretty paranoid when it comes to these things, as you can probably tell.)

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Break point – just before everything blacks out. Note huge quantities of magenta/ blue. The dots are flare spots; normally you don’t see them because they’re relatively dark compared to the rest of the frame. Here, they’re noticeable because there’s so little light from any other source, and the sun is very much in the frame…

In use, the system does double duty: polarisation and variable ND. This is a good and bad thing: bad, because with wider lenses, you land up seeing odd dark bands which are a side effect of applying polarization across a wide field of view*; also bad because you may not want the polarization effect – with water, for instance – but at least you can rotate the whole thing to negate this somewhat. The good is that if you were going to use a polarizer anyway, there’s no need to increase the thickness of your filter stack and risk vignetting – those var ND filters are pretty thick to begin with, and the Zeiss polarizers come in very, very thin mounts.

*Light comes in at very different incident angles, and not all of these are affected by the direction of the polarizer.

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You can get anywhere between two and twenty stops of light attenuation. The lower limit of attenuation is dependent on the base transmission of your polarizer; when they’re both aligned perfectly, there’s no loss from the second one. The upper limit is just before everything blacks out and you have no image at all. However, it’s not quite that simple: at the upper limit, there’s a huge amount of spectral shift; this is caused by the polarizer attenuating the longer (i.e. lower energy) wavelengths of light first; what you see is that the whole image starts to get cooler and cooler, until you’re eventually left with nothing but magenta. Whilst this produces some interesting effects, it’s a bit of a one-trick pony, and cannot be corrected for either with manual WB at the time of shooting, or afterwards with the eyedropper in PS. My theory is that there’s so little of the other wavelengths left that there simply isn’t enough information to make a correction. I think the safe limit is around 15 stops or so; it means that under bright sunshine (think 1/4000s) you can cut things down to as low as 8 seconds. In any case, you can visually see the point at which things start to turn too blue.

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No ND required because it was already dark enough, but I think you get the idea.

The bigger question is of course why: why would you want a variable ND anyway, and why a variable ND over a fixed one? Simple: creative options. Long shutter speeds allow control over motion, or the impression of motion; the ability to precisely control that shutter speed means you can regulate exactly how much motion without having to affect the other exposure parameters such as depth of field. Are there uses beyond the obvious smooth water/ clouds and abstracted people? I’m sure there are, but I haven’t thought of any yet – this probably means it’s time to get shooting again. MT

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Comments

  1. These shots are beautiful. I really like your style.

  2. Marc McVey says:

    NDs are great if you are using manual flashes and shooting wide open. It’s a great work around of you are using flashes that don’t offer FP so you can override shutter sync speed limit of 1/250. Use the variable nd to get your background exposure to your liking and then your flashes can light your target.

  3. If you just want to play around with variable-ND filters (and don’t need it to be high quality, for the sake of experimentation), Amazon (at least in the US) and B&H sell a vari-ND by vivitar that’s quite inexpensive. Cheap enough to be worth buying just to see whether you want to shell out real money for the higher-quality items later. I’m still in the experimenting stage with mine. I suspect I’ll keep the cheap one for rare “fun” usage, but I doubt I’ll spring for the pricier option — I’m just not finding that I use it enough after the initial excitement. As a minor plus, it has a built-in step-up ring so that the filter ends up being larger than the lens’s filter threading (like some of the Nikon-branded polarizers).

    • A sensible approach – but given that the situations under which you’ll need one of these things typically induce flare, I believe it’s worth shelling out for a good one if you plan to do a lot of long exposure work…

  4. El Aura says:

    I’m wondering how smooth an effect one could get with a Nikon V3 (and other Nikon 1 models) that can shoot at 60 fps. If one stacks all images in PS, for really long exposures (multiple seconds) one might get a decent result.

    • That’s an interesting idea too, but a lot of work – and if there’s any tripod movement at all relative to the static bits of the subject, you’re in trouble…

  5. Alun J. Carr says:

    At the risk of sounding like a cheapskate, Polaroid sell a variable-ND filter that is multicoated. At the price, you aren’t going to be looking at B+W quality, and maybe the glass they use isn’t *precisely* as optically flat, or their MC isn’t as good as Zeiss’s T*, but they, or their founder, Edwin Land, *did* invent the polariser, so they might know something about that side of things. At Polaroid’s prices, you can afford to experiment. For a 77 mm MC variable ND, go to polaroidstore.com and search for part number PL-FILFDND77.

  6. Manfred says:

    I have read and been amazed by a lot of articles from you. But this is the first time I’ve seen you reporting something about the use of polarizers. As far as I know, polarizers are one of the very few filters left, that can’t be substituted by any filters in post processing. Therefore I was very interested in your experience and opinion about polarizers since long.
    It seems that especially Zeiss wide-angles employ unnecessary huge filter sizes, as very thin filters are available… E.g. 95mm-filters are pretty costly, thus it seems to be worth to discuss what they can achieve.

    Now, the combination of polarizers let us see the use of polarizers in a new light: amazing results and admittedly this never came to my mind.
    Furthermore for people who advocate, photo technology has vastly reached its summit, variable ND filters would be one more challenge for the industry. Especially with the increasing restrictions of the airlines every saving of bulk and size is very welcome.

    • Yes, polarisers are the one filter that cannot be duplicated digitally (and the only ones I still own). The Zeisses have large filter sizes to avoid vignetting and clear the front element. I believe the variable NDs work on a similar principle.

  7. Godfrey says:

    A minor detail: you needed the reversing ring because you used two circular polarizers. A circular polarizer is a polarizing grate followed by a quarter wave plate … In effect, the grate does the polarizing filtration, then the quarter wave plate depolarizes the filtered light so as not to interfere with sensors and other polarization sensitive elements in a camera that has optical light pipes. However, what this does is make a circular polarizer directional: if both are in line in normal orientation, the depolarized light exiting the first one is not occluded by the grate on the second one in the way you want for a variable ND filter.

    You could simplify your setup by using a linear polarizer in front of a circular polarizer. Linear polarizers are bidirectional so you could just screw one into the front of a circular polarizer and get the effect you want.

    • Very true – except I couldn’t find a linear polariser easily or with the right quality of coatings.

      • Another way of accomplishing the same thing with 2 CPLs would be to remove the glass one of them and turn it around (polarising side towards the back), then mount the glass back. No need for a reversal ring then: just screw the so modified filter on top of a normal CPL.

    • toerag says:

      Godfrey, surely the depolarised light has the reflections reduced in intensity even if they’re then subsequently un-polarised? So two CPL filters will work to an extent? Wikipedia says CPLs only work when viewed from the correct side, thus turning one the wrong way round will negate its effect? I’ll get a second filter to play with anyway, it’s not like they’re expensive.

      • But the first polariser in the correct direction still works as normal with reflections.

      • the use of 2 CPLs (with the first of them reversed) or 1 linear polariser + 1 CPL will give 2 slightly different results. With 2 CPL you really should get something close to a variable neutral density filter. With the other combination, you should get a variable neutral density filter that, depending on the orientation of the linear polariser in the front, will also apply polarisation to the scene.

  8. Hi Ming, I’ve been a looong time reader, but never a commenter, as simply I see a lot of comments on every one of your posts, and I usually don’t have anything valuable to add apart from saying what nice photos you have posted, I especially like your product shots and I try to emulate them myself.

    But not today, I have something of value to add, I just bought an E-M1 and a 12-40 based on your reviews and others, so I needed a new larger 62mm Variable ND filter, mainly for video shooting, and for some long exposures.

    After killing the internet in research, and not wanting to jump for a $300 or more filter, I settled on the 62mm Tiffen Variable ND which I got from Amazon for around $100. All the reviews praised it for great performance for value, especially for video use, so I bought one, and I am extremely pleased with how it works, it only goes to 8 stops, but that’s enough for me.

    I used it last Friday on a fishing trip, and I really like the results.

    • How does it perform shooting into the sun? I looked at the Tiffen but was told that it wasn’t multicoated.

      • I also have the Tiffen, probably as a result of doing the same kind of research mshafik did. I did not like it at all after using it on a 28mm EFOV lens (Sigma 19mm on a Sony NEX-5N): uneven blotchy skies with big color shifts. It might be OK on a longer lens, but that one thing turned me off so much, I haven’t pulled it out in over a year. I’m probably sticking with straight ND filters from now on.

  9. What lens and body did you use to capture that last photo? No signs of front nor back out of focus.

  10. Something I have not tried, but interesting.

  11. That fourth picture in particular is really striking and evocative.

    I’d never presume to demand anything from a site which already has so much superb free content, but if you ever find yourself stuck for something to write about, I think a lot of people would be interested in an article (or a series of them) about how you integrate your knowledge of physics into the way you experiment like this (and to how you shoot in general). I have a great video with Jay Maisel, and he points out that a person’s (educational and life) background will heavily influence the way they shoot. He comes from a background of art and painting, and so he looks for his “holy trinity” of light, colour and gesture. Whereas, if I’m not mistaken, your educational background is in physics and one would therefore imagine that this informs how you approach photography.

    I’m not a very scientifically minded person. Languages are more my thing…don’t know how that affects how I shoot :-), but if it were written from a layman’s perspective, I imagine that a lot of people would be interested in reading such an article.

    Anyway, just an idea, certainly not intended as a demand!

    • Actually, I’ve been meaning to write something like that for a while – just haven’t had the time…but yes, you’re right: it does influence my approach.

  12. Paul Stokes says:

    Liked 1 and loved 4. Certainly an impressive colour palate. Keep experimenting.

  13. Kristian Wannebo says:

    I love nr. 3 and nr. 4!

    Nr. 3 : The colours are amazing, and the flares add more life.

    Nr. 4 : I had no idea water could look like that with a 4s (?) exposure, there is still so much fine structure near the shore.

    Nr. 1: If I didn’t know (or guess) this was a long exposure, I would think of ice in spring.

    • Thanks – there’s a fine line between too long an exposure with water and too short (no nice streaks) – it always generally moves in lines back down towards the body of the sea, which is what creates the structure.

      Ice is still water – just not moving at all! :)

      • Kristian Wannebo says:

        Exposure: I can imagine that.
        I was wondering about the sharp looking fine structure in the white patches of foam, 4s(?).

        Ice: Yes, but it never looks the same.
        And, especially when it has thawed and frozen again a few times, it can look almost this strange.
        Just like water, it’s a strange beast.
        And when it freezes, it sings.

  14. Hi Ming. Thanks for such providing such a thoughtful and informative site. I do mostly video and these Genustech Eclipse’s are fantastic variable ND filters that don’t degrade image quality after about 85mm and longer. I do not work for these guys :)

    http://www.genustech.tv/collections/genus-eclipse-nd-fader

    rankglencairn.wordpress.com/2012/12/08/nd-fader-shootout-heliopan-vs-the-new-genus-eclipse/

  15. These are amazing, the texture gives it an almost 3 dimensional effect.

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