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View Full Version : Has anybody tried this, variable nd


MotoCroz
28th February 2011, 11:06 AM
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.:confused:

benvendetta
28th February 2011, 12:56 PM
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.

MotoCroz
28th February 2011, 01:27 PM
heres the link to the post describing the idea.
http://www.digital-photography-school.com/create-your-own-variable-neutral-density-filter

and heres a video demonstration
http://www.youtube.com/watch?v=Jmwpi0RRuyo

Ellie
1st March 2011, 02:21 PM
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.

photo_owl
1st March 2011, 08:45 PM
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

benvendetta
1st March 2011, 10:41 PM
So it doesn't work with two circulars then?

Ellie
2nd March 2011, 01:25 PM
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.

photo_owl
2nd March 2011, 04:02 PM
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.

Barrie Norman
2nd March 2011, 05:23 PM
AF will not work!

photo_owl
2nd March 2011, 06:19 PM
AF will not work!

under what circmstances - which body, which AF system?

David M
3rd March 2011, 02:27 AM
AF will not work!

And that matters why?

photonutter
3rd March 2011, 09:25 AM
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.

LightingMan
3rd March 2011, 09:57 AM
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.*yes

David Morison
3rd March 2011, 12:46 PM
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.

Regards

David

Barrie Norman
3rd March 2011, 05:37 PM
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.

Ian
3rd March 2011, 06:02 PM
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.:confused:

Wasn't my article by any chance? :)

http://dpnow.com/7300.html

Ian

David M
3rd March 2011, 11:32 PM
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.

I know why AF won't work with a linear polariser, I just don't see why it matters.

If you're using stacked polarisers for a strong ND effect you're usually after longer shutter speeds. So you'd normally be tripod mounted and set to MF anyway.

MotoCroz
4th March 2011, 10:09 AM
Wasn't my article by any chance? :)

http://dpnow.com/7300.html

Ian

No sorry Ian it was the one in my second post but yours isnt as condecending.*clap
As for the focus one item ive read said so long as the circular one was nearest to the camera it should work but my first link recommends turning the filters till they give the brightest image, spot focusing then turn off the auto focus and adjust the filters to what you want.
What I would like to know though is the colour image usable or is it basically a B&W thing.

Ian
4th March 2011, 11:00 AM
No sorry Ian it was the one in my second post but yours isnt as condecending.*clap
As for the focus one item ive read said so long as the circular one was nearest to the camera it should work but my first link recommends turning the filters till they give the brightest image, spot focusing then turn off the auto focus and adjust the filters to what you want.
What I would like to know though is the colour image usable or is it basically a B&W thing.

My experience is that colour tends to be distorted. And you can get variable density across skies and other areas of the scene which requires careful adjustment. Black and white is the easy way to go, but colour should be possible with some work.

Ian