Hello friends and neighbours. In this article I want to chat briefly about Neutral Density filters, specifically the critter called the Variable Neutral Density filter.
This sounds like a great idea. A single screw-in filter that can deliver a variable amount of light cut across a range of stops. What could be easier and more convenient?
It's Not Really a Neutral Density Filter At All
Regular fixed neutral density filters started out as a light reduction gel sandwiched between two pieces of optical glass. Later on, in better filters, these optical glass elements received multi-coating and hardening and the gels were also treated to reduce internal reflections, colour alteration and to get better consistency. Some makers even went the dyed glass route as opposed to the sandwich route. So long as the glass was good, was properly multi-coated and when gels were used, they were of high quality and consistency there really was no problem at all.
Someone realized that a polarizing filter cuts light, and that two polarizers in opposition would cut different amounts of light depending on the orientation of the two polarizers relative to each other, and there was born the Variable Neutral Density, which is not a neutral density filter at all, but instead two polarizers working against each other.
Good Polarizers Are Expensive
As any serious image creator knows, there is a big difference between a really great Polarizer like a B+W Kasemann or Heliopan and the craptastic ones sold under fake names in camera stores and in online marketplaces. We find a lot of really lousy polarizers coming out of the Far East with multiple brands on the same piece of crap. I'm not being elitist, I'm telling the truth. A cheap polarizer is going to be junk, and it would better for you to cut the bottom off a coke bottle and spray paint it clear grey. You would probably get better quality images.
Don't buy cheap shit. You'll just regret it.
As an offside, here's my favourite polarizer. It's the B+W Kaesemann XS-Pro MRC and if you click the image, you can buy one from B&H Photo Video. Note that the link is for the 82mm which is what I needed, and I just use step down rings to put it on lenses with smaller filter diameters.
If Good Polarizers are Expensive and a Variable ND Needs Two....
Why then can you buy a variable ND so inexpensively? Because they are cheap shit. A good variable ND is going to cost almost as much as two very good polarizers, and if it doesn't, well there's the flashing red light and siren warning you that you are about to get a piece of crap.
Thus we find the general bad reputation of the Variable Neutral Density. The bad rep is because so many of them are horribly bad.
How Do You Know a Bad Variable ND
- A bad variable ND will display one or more of the following characteristics
- Nasty colour shift, mostly towards the green of rotted lettuce
- Massive amounts of moire, specifically the visible evidence of optical interference patterns
- The appearance of a soft X in the frame where the X part of the frame is of a different exposure than the non-X, particularly towards the edges
The wider the range that you seek in a variable ND the faster these detriments will appear and that's why we find the best variables never go beyond 5-6 stops of light cut. If you find a 10 stop variable, you have discovered junk.
Trends in Neutral Density Filters
in the last few years we have seen a jump in demand for massive amounts of light cut, either to create super long shutter open times, or to allow for very shallow depth of field in very bright light. The Lee Big Stopper at 10 stops was the first of many to hit the popularity pipeline. Even as good as it is, it did exhibit a colour cast, and while this was easily corrected in post processing, Lee changed their lineup completely to a new design called IRND that avoids the colour shift.
We know see fixed NDs from 1 stop to 15 stops of cut. There is even a 15 stop variable that has been announced by the maker of an electronic slider, and goodness knows that a maker of mechanical sliders would be best qualified to make a 15 stop variable. Or to rebrand some hunk of crap.
Large stop rating Neutral Density filters while very useful will often cause your autofocus to throw up its hands and give up. Instead of giving up, your in camera light meter takes a whack of heroin and starts throwing darts at the exposure chart. If you are not prepared to prefocus and to calculate your exposure with an external tool, a very dark ND is probably not for you in the first place. if you are however, mmm tasty!
Picking a Good Variable Neutral Density
I personally use the Heliopan Variable Neutral Density which was well over $400 CAD when I got it and it works a charm. I know a fellow who bought one and hated it. Your mileage may vary. I also have experience with the B+W XS Pro Variable. The B+W is 1-5 stops and the Heliopan is 1-6 stops. I have heard good words from peers that I trust about the Singh-Ray Variable Neutral Density filters but I have never shot one. I have tried lots of others, and the only one that came close to the two that I tried was the Tiffen Variable that I use on my pro video camera which shoots a Super 35 sensor on a full frame 35mm image circle. That level of cropping in, cuts out the subtle X occurrence of the Tiffen. It's hard to see, but I am a picky sort.
If you do buy a variable from one of the two good companies, pay close attention to the ring depth. Because this is a deep filter, you may get some vignetting on ultra wide lenses. While this can be adjusted for in post, it can be very annoying, particularly if you are using very small apertures.
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.