Variable Density Filters

Discussion in 'Digital SLR' started by wheel, Nov 27, 2006.

  1. wheel

    wheel Guest

    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.

    Are there any advantages to a combo unit like the Vari-ND besides
    convinience (?) and the index?
    wheel, Nov 27, 2006
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  2. wheel

    frederick Guest

    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.
    frederick, Nov 27, 2006
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  3. 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 Littlewood, Nov 28, 2006
  4. wheel

    Toby Guest

    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, Nov 28, 2006
  5. wheel

    wheel Guest

    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
    wheel, Dec 1, 2006
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