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Accessory talk Those important extra system components.

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Old 28th February 2011
MotoCroz MotoCroz is offline
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Has anybody tried this, variable nd

Just read an article showing that if you put a linear polarizer on top of a circular pl filter you get a variable ND filter. Seems too good to be true, so as anybody tried it.
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Old 28th February 2011
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Re: Has anybody tried this, variable nd

I think that I only have circular polarisers, and different sizes at that.
Not sure if this would work.
I would like to know a bit more about the Lightcraft fader ND filter. They don't appear to give the full range of filtration at all focal lengths so are not quite as versatile as they first appear. I was planning on getting one at Focus next week but may just get a 9 or 10 stopper instead.
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Old 28th February 2011
MotoCroz MotoCroz is offline
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Re: Has anybody tried this, variable nd

heres the link to the post describing the idea.
http://www.digital-photography-schoo...density-filter

and heres a video demonstration
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Old 1st March 2011
Ellie Ellie is offline
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Re: Has anybody tried this, variable nd

It seems an interesting, but quite logical, thing to happen because the different types of polarisers will compete with each other and at some point will block out all available light.
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Old 1st March 2011
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Re: Has anybody tried this, variable nd

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Originally Posted by Ellie View Post
It seems an interesting, but quite logical, thing to happen because the different types of polarisers will compete with each other and at some point will block out all available light.
used to do this all the time in 'the old days' with a pair of linear's - all of my modern ones are CPLs
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Old 1st March 2011
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Re: Has anybody tried this, variable nd

So it doesn't work with two circulars then?
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Old 2nd March 2011
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Re: Has anybody tried this, variable nd

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Originally Posted by benvendetta View Post
So it doesn't work with two circulars then?
It will only work if you can get the polarisers to, effectively, compete with each other, which isn't likely to happen with a pair of circular polarisers because they're both working in the same direction.

You might be able to get it to work with a lens from a pair of polarising sunglasses.
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Old 2nd March 2011
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Re: Has anybody tried this, variable nd

I've only tried it with a couple of mine and it results in some interesting colour effects overlayed on the fading - magenta and violet being the main ones.
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Old 2nd March 2011
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Re: Has anybody tried this, variable nd

AF will not work!
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Old 2nd March 2011
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Re: Has anybody tried this, variable nd

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Originally Posted by Barrie Norman View Post
AF will not work!
under what circmstances - which body, which AF system?
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Old 3rd March 2011
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Re: Has anybody tried this, variable nd

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Originally Posted by Barrie Norman View Post
AF will not work!
And that matters why?
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Old 3rd March 2011
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Re: Has anybody tried this, variable nd

Quote:
Originally Posted by photo_owl View Post
I've only tried it with a couple of mine and it results in some interesting colour effects overlayed on the fading - magenta and violet being the main ones.
The colour variation will depend on brands of filter and their effective infrared/ultraviolet filtration.
A daylight filter will correct if required.

Edit,
It does work with two circular, but somewhat less predictable than one linear. You may need to reverse the glass in one.
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Last edited by photonutter; 3rd March 2011 at 10:10 AM.
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Old 3rd March 2011
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Re: Has anybody tried this, variable nd

Quote:
Originally Posted by Barrie Norman View Post
AF will not work!
Whenever I'm using a ND filter I'm typically looking for a longer exposure. When I'm looking for a longer exposure I typically have more time; to set the equipment up, using a tripod or beanbag, pull out the Thermos flask etc. So being able to shoot from the hip with AF isn't really that important in these circumstances.

But my quizzical mind would really like to know why is doesn't work. I assume it's something super technical involving refraction of light, antimatter and matter sharing the same sensor and stuff.
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Old 3rd March 2011
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Re: Has anybody tried this, variable nd

Using two polarising filters with one fixed and one rotatable used to be a common method in high-end micro-photography to provide a continuously variable ND effect and I am sure it will work with DSLR photography.

Please excuse me if I appear confused but aren't all polarising filters linear, isn't that why they block reflected (polarised) light only in one plane and need rotating to block reflected light in another plane. Surely a CPL is just a linear filter, with a quarter wave plate on the lens side for AF purposes, that can be rotated on the front of a lens to achieve the effect desired? The effect on the depth of colour in skies etc. is because of the amount of polarised light from these subjects, which is why rotating the filter changes the effect, this could only work with a linear filter. Weren't CPLs designed to rotate because the front mount of lenses sometimes rotate with focussing or zooming? In this case two CPLs could work as long as the rear one can can be fixed, but I don't know what effect the quarter wave plate on one will have.

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Last edited by David Morison; 3rd March 2011 at 12:59 PM. Reason: Addition
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Old 3rd March 2011
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Re: Has anybody tried this, variable nd

Quote:
Originally Posted by David M View Post
And that matters why?
The auto focus on any DSLR will only work with a circular polarising filter by stacking a linear and a circular will not allow the AF to work, for the same reasons as using a linear polariser alone it is something to with blocking the light from he AF sensor, there are a lot of articles which will explain this better than me. however I do use this method on my film gear but it is hard to work out the exposure without TTL metering, fortunately I have a metered prism to get over this.
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