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Possible Duplicate:
What is the difference between using an ND filter versus 2 polarizers?

One thing I was always wondering when it comes to ND filters is why there's need for such a large amount of different strengths, when two linear polarization filters would do the trick just fine. Assuming a light loss of 1/sqrt(2) per filter, one could easily create a ND "1 to infinity" filter. Why is that not in everyone's camera bag?

Unfortunately, all the filter companies offer only LCPL (linear, then circular), so I can't simply make one myself. Apart from cases where I do not want to take polarized light into account while having a ND filter (architecture photography maybe), I can't think of a reason not to do this.

Edit: More specifically, I'd like to know why there's nothing like this at all, or how I could get my hands on such a filter.

Community
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David
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2 Answers2

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Besides Singh-Ray, which are extraordinarily expensive, Tiffen and Kenko and others also make variable ND filters. Some call them "faders". The problem with them compared to fixed ND filters is that they can create strange color casts and banding (even with the top of the line Singh-Rays) and as Stan said, being composed of polarising filters they have uneven effects across the image when using wide angle lenses, especially in skies.

MikeW
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Does Singh-Ray's VariND filter not count?

Note, though, that even in their own product advertising, they point out the fundamental problem with this filter arrangement: it's still a polarizer, so it's still going to give unsatisfactory results on skies when used with a wide-angle lens. And there's that little thickness problem as well; vignetting is going to be an issue on wide lenses or on lenses that don't have a lot of wiggle room between the front element's edge and the filter thread.