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.
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.)
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.
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.
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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