Variable ND Filters
In the early days of the DSLR revolution the pixel peepers and measurebators all trashed the variable ND filters. I was amazed when I first heard about them–didn’t know there was such a thing. I avoided them because I had heard about how they soften the image, etc. I stuck with my high quality B+W ND .3, .6 , and .9.
Recently a friend had an out of town shoot. The only ND he had was a 72mm .9, which got him through most bright sun situations. He almost never took off his 28-135 lens from his 7D. However, he knew that on the trip out west he would need to shoot with his wide angle zoom, which had 77mm threads. No time for him to order from B&H so he went to one of the few remaining local camera shops. Naturally they had no 77mm filters–except for one variable ND. It was a low end Genustech: http://www.bhphotovideo.com/c/product/676824-REG/Genus_GL_GNDF_77_77mm_ND_Neutral_Density.html
Of course, at the local camera store it was about $200, but the dude didn’t have any choice. When the Heliopan, Schneider and other respectable variable ND filters cost $400 and up, this one seemed a serious compromise to me. Imagine my surprise when all his footage looked great.
If I did some serious measurebating, I’m sure a side-by-side comparison would show me that this filter isn’t great. But it’s good enough for what this guy does. I was willing to spend more for mine and got the Schneider: http://www.bhphotovideo.com/c/product/851445-REG/Schneider_68_031177_77mm_True_Match_Vari_ND_Filter.html
The reason I got the Schneider over the Heliopan or Singh-Ray was because the Schneider has 11 stops of ND (“It goes all the way to 11!”). This filter was for use on my FS100, whose “native” ISO is 500. You don’t walk outside without an ND filter shooting at 500 ISO. And, I’ve shot some interior interviews with the ND on as well. I figured that to shoot fairly wide open in bright sun I’d need to lose 9 or 10 stops. With any variable ND, regardless of cost, you don’t want to use the most open or most closed position because of some funkiness that’s introduced. Having 11 stops gave me the slop I’d need for the worst case scenario.
So, why did I spend 400 bucks on a variable ND instead of simply adding an additional ND to my filter inventory? Two reasons: Control…and more control.
The variable ND does two things quite nicely. First, it eliminates the dreaded click stop problem we get when using still camera lenses on a video camera. If I want, for example, to follow a subject from a shady spot into the bright sun, I’m going to have to stop down as I pan. Can’t do that with still camera lenses because of the click stops. Cine lenses don’t have click stops, and if you have old Nikkor lenses you can send them to Duclos for click stop removal. With Canon lenses you’re stuck with click stops.
With a variable ND filter, click stops are no longer a problem. I can simply turn the filter as I pan and the exposure changes as smoothly as if I had a Zeiss cine lens.
Here’s the other nice thing: the exposure changes without changing the lens aperture. When reducing exposure with the aperture, you’re stopping down the lens, increasing depth of field. Not really a bad thing in most cases, but here’s how doing it with a variable ND helped me out recently. I was shooting an interview in a small office. I needed the 35mm Nikkor lens open to f2 in order to get the shallow DOF I wanted to keep the books on the shelf in the background out of focus. I didn’t want to be able to read the titles. The S35 size sensor in the FS100 needs about a 2.8 or more to do the same thing, DOF-wise, the 5DII can do at a 3.5-4.
At an f2 the scene was overexposed. No problem, just rotate the ND until the exposure is perfect. No need to be concerned with the half stops of the lens being too much or too little–with the filter I can dial it in with precision. If I had put on my ND.3, it would not have been enough, and the .6 would have been too much. That’s the beauty of the variable–you simply rotate the ring until the exposure is perfect.
If you go to the B&H variable ND filters listings, you can find one for just about any price consideration. http://www.bhphotovideo.com/c/search?Ntt=77mm+variable+nd&N=0&InitialSearch=yes&sts=ma&Top+Nav-Search=
I feel the same about filters as I do lenses–I want good ones. If I have a nice lens, I don’t want a cheapo filter on it. That’s just my opinion and using a cheaper filter may be fine in many cases. If you have a $400 lens it might be a bit much to buy a $400 filter for it. If you buy from B&H, keep all the packing materials and the box in pristine condition and you can return the filter if you don’t like it.
Another nice thing about a variable ND filter is that it allows you to more easily use your lens in its “sweet spot.” Many lenses seem to work best in the mid ranges, between f5.6 and 11, for instance. With as variable ND you can set your f-stop where you want and adjust exposure easily simply by turning the filter ring, just as you would a polarizing filter. If you’re doing a big budget motion picture you’re not going to be using still camera lenses and you’re not going to use a variable ND–you’ll use high end prime lenses and the best filters available. But for micro budget indie filmmaking, documentaries, corporate work and so on, I think a good quality variable ND is fine. I doubt that anyone would ever notice any difference if I switched between my Schneider variable and my B+W normal NDs.