Linear + circular polarizers for ND

[Greg D]

I've seen it suggested before that a linear ploarizer on top of a circular one makes a good variable neutral density filter.  What are the drawbacks to using this method?  I would assume there would still be the issue of darkening in the top center of the frame that one normally sees with a polarizer, correct?  Other problems?  Sounds too easy.......

[francois]

I've seen it suggested before that a linear ploarizer on top of a circular one makes a good variable neutral density filter.  What are the drawbacks to using this method?  I would assume there would still be the issue of darkening in the top center of the frame that one normally sees with a polarizer, correct?  Other problems?  Sounds too easy.......

You'll probably have vignetting as the two stacked filters will be thick enough to be problematic on wide angle lenses. While I haven't seen the Singh-Ray Vari ND filter ( http://www.singh-ray.com/varind.html ), I believe that it's essentially two polarizers (circular & linear) stacked together. I might be wrong?

[Hank]

I've done a fair bit of it, using a step-up ring from a smaller filter on the lens to a larger on the ring to overcome vignetting when I'm shooting a WA lens.  

The biggest problem I've encountered is color shift.  You'll get a decided green cast...... okay, make that a sickly olive cast..... if you mix brands of filters, and especially if they're cheaper filters.  Best I've tried for beating the color cast is B+W.  

It's not a perfect system, but good enough to try when conditions warrant-  if you already own the filters and happen to have them in your camera bag.  If buying new filters for the job, I'd forgo the extra polarizers and move directly to a variable ND.

[francois]

I've done a fair bit of it, using a step-up ring from a smaller filter on the lens to a larger on the ring to overcome vignetting when I'm shooting a WA lens.  

The biggest problem I've encountered is color shift.  You'll get a decided green cast...... okay, make that a sickly olive cast..... if you mix brands of filters, and especially if they're cheaper filters.  Best I've tried for beating the color cast is B+W.  

It's not a perfect system, but good enough to try when conditions warrant-  if you already own the filters and happen to have them in your camera bag.  If buying new filters for the job, I'd forgo the extra polarizers and move directly to a variable ND.

FWIW, David Mantripp (drm: profile), reports that the Singh-Ray Vari ND also suffers from color shift.

Here's the link to his blog: http://www.snowhenge.net/index.php/pblog/2008/03/

However, I cannot recommend the Singh-Ray filter, as it introduces some very nasty colour shifts and irregular darkening. This is partly due to the design, based on two polarisers, but frankly, it is way over-priced and clumsy to use. It seems worse on the E-3 than the E-1 or E-400, but it isn't brilliant on them either.

[Hank]

If it's based on two polarizers, fuggedaboudit.  Been there done that.  I'd simply go back to packing ND filters before I'd spend extra for it.  Using two B+W's does help eliminate the color cast, but at the same time colors get a little "muddy" for lack of a better term.  You'll end up bumping contrast, or better yet using the black controls on each color in Photoshop's Selective Color adjustment.

[MarkL]

I've seen it suggested before that a linear ploarizer on top of a circular one makes a good variable neutral density filter.  What are the drawbacks to using this method?  I would assume there would still be the issue of darkening in the top center of the frame that one normally sees with a polarizer, correct?  Other problems?  Sounds too easy.......

Wouldn't 2 polarizers of any type do the job? It would be one heck of an ND filter, you'd lose 3-4 stops before even changing them relative to each other.

[Hank]

In my experience you'll do fine with two linears, but two circs doesn't work nearly as well as other combos.  You do get the 3-4 stop light loss as a minimum with two circs, but I don't like the results.  As an aside or an elaboration, I've had better results in that range using a 2-stop ND filter along with a circ.