I think the Singh-Ray Vari-ND variable ND filter sounds great, but is really expensive.
There are way cheaper variable ND filters from some chinese sites.
I wonder: are the cheap ones worth the money? Is the vari-ND worth the premium you pay for it?
or: what's the difference between cheap knock-offs and the Singh-Ray?
The mechanism is likely a combination of a linear polarizer and a circular polarizer (which itself is a linear polarizer followed by a quarter wave plate). Thus the differences you can expect between high-quality and cheaper variable ND filters ought to be similar to the differences found among polarizers which include flare, vignetting, inhomogeneity, and color shifts. You can avoid flare in many cases by not including bright lights in the photo; vignetting can be compensated in software and sometimes is desirable; I suspect the inhomogeneities are usually not noticeable; but color shifts can be pronounced. Typically a set of crossed polarizers (which is what you're using to get the heaviest ND settings) pass essentially no colors except violet. Therefore, I would expect cheaper variable ND filters to work OK except at the heaviest settings (more than about 4 stops) where the violet shift will become pronounced.
You can see evidence of vignetting and a violet shift on this dealer's website. Compare the bottom two photos: the one taken with their ND filter has pronounced vignetting (many stops) and a strong blue cast. In contrast, look at the photos on the Singh-Ray site. If you look closely you can see the vignetting, but there's little evidence of a violet shift: look at the grays of the rocks in the stream. Maybe those were post-processed to remove the blue, so the comparison is not definitive, but these two sites nicely illustrate the difference we would expect between good and bad variable ND filters.
I did a quick test using a high-end, top-rated Marumi circular polarizing filter and an old low-end Vivitar linear filter. In combination they made a fine variable ND filter, although there were some color shifts (first to yellow, surprisingly), but below about 3-4 stops a yellow incandescent light started to take a distinctly blue hue. At the maximum density, probably around 6 - 8 stops (I only looked, I didn't measure), only the light was visible and it was a brilliant blue, the color of a Wratten 80b filter.
Your best bet for a cheap solution, then, is actually to look through one of the inexpensive filters and pay special attention to the apparent color at the densest ND settings. While you're at it, look at a bright light through the filter to check for flare and scattering. If everything's acceptable you probably have a great deal.
http://en.wikipedia.org/wiki/Neutral_density_filter
"This can sometimes create color casts in recorded images, particularly with inexpensive filters."
I haven't used the Singh-Ray Vari-ND but I do own Singh-Ray (and LEE) split neutral density filters and a B+W 10 stop ND filter. I found the LEE SND filters gave just as good results as the Singh-Ray in my opinion however the Singh-Ray was larger and I could use it on my 4x5 lenses. I did at one point buy a cheaper 10 stop ND filter and wound up getting kind of a murky yellow/brown/gray cast to everything while the B+W filter didn't add any casts.
I love the B+W filter btw.
As a side note/opinion, given the extreme cost of Vari-ND filters I'd suggest looking at just getting a normal ND filter.
I don't have any cheaper variable ND filters to compare with. But I do have other filters including some smaller ND's from various brands. I also have the Singh-Ray Vari-ND on my desk which I have not been able to use yet. Just going on build quality I am in love with the Vari-ND. It is well machined etc. I do see a small amount of color shift when looking through the glass when held up to my eye. But that is to be expected. Another issue with the build quality of the cheaper filters is that the glass is sometimes loose. You can feel it shifting in the holder. Is it a huge problem? no but I have seen them come apart over time.
The reason I went with the Vari-ND is its ability to be quickly opened up so that you can focus, compose etc. Then you can quickly stop it down for the shot. I use all L series glass and most of it except the 24-70 are internal focusing as well as internal zoom. So there is less likely to shift your focus when adjusting the ND. But that is an issue on some lenses to consider. When you add and remove filters you can shift things. My idea was go with the variable to reduce the changes to a minimum.
Why mention that? Again part of it is the build quality. Usually the cheaper brands are made of cheaper metals etc. I have a few that were given to me that cannot be used any longer due to chewed up threads from where they were cross threaded, dropped, or just beat up from use. The Singh-Ray seem to be more durable and, recessed past the edge of the filter for protection.
I may still have a lemon as far as shooting goes. But I tend to buy decent quality gear first before spending a lot more money buying several of the cheaper knock-offs. We will know more when I can get out and shoot with it. Hopefully this weekend.
I bought a cheap ND8 filter from a chinese website and it had a HUGE megenta colour shift. Very pronounced at all settings.
