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Variable ND filters

Discussion in 'Lens Matters' started by gray1720, Nov 15, 2019.

  1. gray1720

    gray1720 Well-Known Member

    Has anyone out there used one of these? If so, any issues or problems you've come up against using them?

    I'm interested as they are obviously cheaper than a fixed density filter, and look as though they will be usable in a wider range of light conditions than a fixed density one, but as they are more than one extra piece of flat glass (as far as I can see they are a denser polarising filter), how much do they degrade an image?

    TBH I'm not sure I want to spend a huge sum of money on something I intend to use to make a few images every now and then a bit more interesting, rather than as a serious tool.

  2. PeteRob

    PeteRob Well-Known Member

    I've got one (a gift) but I've never used it. I'm away for a few days but could do some tests when I get back if that helps
  3. gray1720

    gray1720 Well-Known Member

    Yes please, Pete, that would be great. I'm thinking about using one to get more ball movement in my cricket photos (obviously I'll need some kind of support too) after the thread with the blurry photos a while back.
  4. PeteRob

    PeteRob Well-Known Member

    A polariser will lose 5/3 of a stop
  5. Petrochemist

    Petrochemist Well-Known Member

    Variable ND filters are as far as I know all made by combining two polarisers so that one can rotate relative to the other
    When both are aligned it effectively works as a single polariser, when perfectly out of alignment (crossed) each should block half the light leaving none. I reality some light always remains but crossed polarisers can drop light transmittance by significant amounts.
    They are not an ideal solution especially with wide angle lenses - where the polarisation of the sky often introduces a dark X.
    In some cases this X can totally overpower the image. :(

    Most polarisers transmit NIR, so I've used them with my converted camera to give a controllable balance between visible light & NIR. For most photographers this feature will be irrelevant, but some cameras have weak IR cut filters that will see IR through crossed polarisers.

    I don't know where you get that from!
    The amount of light a polariser removes depends on the polarisation of the scene and the orientation of the filter. If the scene is randomly polarised a perfect polariser should lose 1 stop, but in real life a clear sky is polarised when at 90° to the sun, and reflections are generally polarised too.
  6. PeteRob

    PeteRob Well-Known Member

    That's what I get. My MF film camera has no meter so I have to compensate. Based on readings with and without on a 35 mm camera. It is an ancient 67 mm filter by Cherry, found in a bits box of a defunct camera shop

    Just googling, Lee filters say loss up to 5/3 stop with theirs. Wiki says from 5/3 to 2 stops for CPL based on Hoya and Cokin sources. I found a review of the Hoya (77 mm digital pro) I use on my digital kit that says measured at 4/3 stops but I don't use a hand-held meter with the digital kit.
    SqueamishOssifrage and peterba like this.
  7. peterba

    peterba Well-Known Member

    By coincidence, Pete, a short while ago I did some testing of my polarising filter (a fairly basic Hoya one), because likewise, my MF has no meter, and I wanted to know how much compensation to use.

    My testing showed that a small amount of variation, depending upon the setting of the polariser, but for practical purposes, my findings matched your figures i.e. simply allow 1⅔ stops of compensation, and the job's done.
  8. gray1720

    gray1720 Well-Known Member

    So... we've established that they are a variant on polarisers, especially as the instructions all say "Don't turn too far or you'll get a black X".

    Obviously, their effect on a wide angle lens is going to have to be considered on my ultra-wide 400mm (especially as I have a crop-factor sensor, cutting the field of view down even further).

    So... with the honourable exception of Pete no1,... what was the question again?
  9. PeteRob

    PeteRob Well-Known Member

    I'm just leaving now - but for when I get back and testing - how many stops light loss do you look for ? I'll use a 400 - I think the filter size on mine is 77 mm (like the filter).
  10. gray1720

    gray1720 Well-Known Member

    The ones I've looked at claim up to 5 stops - I suspect the image quality gets worse the further you go - but what I would use will depend on the avilable light. Sorry, that helps you not at all!

    I guess see how far you can get and still get usable images?
  11. Petrochemist

    Petrochemist Well-Known Member

    So one source says up to 5/3 & another claims 5/3 to 2 conflicting advice & both wildly out in special circumstances. If most of the light one a scene is polarised (such as light reflected of water) in one orientation the polariser will block 99% of that light giving a drastic reduction in light, turned through 90° the polariser will let through all the light so will have practically no effect on the exposure.

    Experiment photographing a LCD display & you should get a very clear example of this. It's easy to turn a white LCD black with a polariser.
    Here's an example with something else added to the shot for interest:
    [​IMG]stressed stencil by Mike Kanssen, on Flickr
    (Lighting via a white LCD screen directly behind the stencil, & photographed through a polariser turned to make the screen black)
    EightBitTony likes this.
  12. gray1720

    gray1720 Well-Known Member

    @PeteRob did you ever get a chance to try this, or was it you whose break fell victim to Norovirus?

  13. PeteRob

    PeteRob Well-Known Member

    I haven't forgotten. Combination of being away, being busy and rain. No norovirus thank goodness.
  14. gray1720

    gray1720 Well-Known Member

    It was obviously someone else then! A blessing for you, a bugger for them.

    No worries, the cricket season is hardly looming!

  15. PeteRob

    PeteRob Well-Known Member

    The fact cricket in this country is away off is a factor! I wanted to try it with a 400 mm in a similar context (subject distance, light) so is dependent on me getting out to do it. My garden isn't suitable.
  16. gray1720

    gray1720 Well-Known Member

    There's probably a take-home message for me in taking it seriously! Thank you for doing so.
  17. Chester AP

    Chester AP Well-Known Member

    From reviews I've read in AP, the variable ND filters are two polarising filters that can be rotated independently so that the effect varies from 'a bit' to 'a lot'. And also, you might not want any image changes caused by the polarising filters.

    Also, you need to consider which lens(es) you might use the filter with.

    When I purchased an expensive 77 mm polarising filter to use on a Sigma 10-20 zoom, I found it useless at the wider end of the zoom because of the large angle of view, which is something AP always forget in their regular 'essential filters' features (the effect is uneven across the image). For much less money I found a decent glass graduated ND filter which experimenting suggests is about '2 stops' (ND4) and the darkest point fading to clear at the centre of the filter. It is now left on the lens for 99% of the shots taken with it - you can get a lot of bright sky in a shot with a lens like this.

    Parhaps you could buy secondhand ND4 (2 stops) and ND8 (3 stops) filters , which would give you the flexibility of 2/3/5 stops as required. When you say 'they are obviously cheaper than a fixed density filter', are you sure you are comparing filters of similar optical quality?

    If you really want to experiment at minimal cost, LCE have a secondhand variable 77 mm ND for £4.99. Just add a cheap stepping ring if required. If you don't like the results, you've found out before spending too much money.

  18. gray1720

    gray1720 Well-Known Member

    Ooh, I might just do that - SRB will do a step-up ring for the same cost so if P&P isn't too silly... thanks!

    I don't think I'll have any issues with wide angles as it'll be on a 400mm! And I'm quite sure I'm not comparing filters of similar quality, but I am a cheapskate looking for something that'll come out a few times a year rather than a serious bit of kit.
  19. gray1720

    gray1720 Well-Known Member

    PeteRob, I've bought the filter suggested above, but would still be interested to compare my results (when I get some...) with yours, if that's OK? I am an eternal muddler (despite being a scientist by trade...) so you may well find out things I miss by fiddling.

  20. Chester AP

    Chester AP Well-Known Member

    Another bargain from LCE.

    If you use it to darken skies, compare the result with this - it's one of the first shots I took using the ND grad on the Sigma 10-20 after abandoning the PL filer. I have a 40 x 60 cm framed print on the wall, but would love to have it larger still if there was room.

    Attached Files:

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