Variable Density Filters

Is a variable density filter like the Singh-Ray Vari-ND essentially two
polarized filters in rotation? If so can I attain the same effect by
adding another 77mm CPL to my existing one? I've read as much, and it
matches my rusty understanding of paired polarizing filters, but wanted
to ask here for opinions.

http://www.singh-ray.com/varind.html

Are there any advantages to a combo unit like the Vari-ND besides
convinience (?) and the index?

 

When you rotate two polarisers, it ends up with a large colour shift to
blue - way beyond correction in PP. This probably is two polarisers,
but they seem to claim that colour isn't affected.
Jeesh - $340/$390! Seems a useful bit of kit - if it works.
The advantages would be that you can compose then rotate and take the
image - although this is also simple with drop-in (rather than screw on
ND filters) The disadvantage I guess is that with ND filters you can
meter manually with the filter out, then drop in filters of a known
density and adjust exposure by a known number of stops.
I'm inclined to believe that if something seems too good to be true then....
So, could you please buy one and report back, posting some unedited full
size samples
The "larger" sample images on the site are still very small.

 

It does indeed look like two polarisers.

I have a Zeiss unit (a microscope accessory) which has two polarisers
which can be rotated relative to each other, and it is a very effective
variable ND filter. However, as frederick says, even with high quality
Zeiss filters, it does give a very strong blue colour when set to
maximum density. Its not too bad at lower densities though.

Note that your idea of adding a second circular polariser is doomed to
failure; the light from the first one will come out elliptically
polarised and will not be varied by the second one. You need to use a
linear polariser at the front; a circular polariser at the rear will be
fine, if you need to avoid metering/focusing problems.

David

 

Perhaps the high price is due to using special pol filters that do not
exhibit your blue shift. I have seen industrial pol filters that do not
shift color. All of my CP photo filters do exhibit it, however.

Toby

 

Thanks to the three of you, useful info, not sure what I'll do yet but
am leaning towards buying one of these units. I'm kind of surprised that
there don't seem to be other like-kind variable density filters on the
market (at least so I guess from the newess of this type of item to
you).