# Using a polarizing filter with nd filter for long exposures



## bereninga (Feb 2, 2015)

I'm playing around with some long exposures and wondered if circular polarizers should be used with an nd filter. At 24mm, I get really bad vignetting. It looked like the filter rings were actually visible in the shots of 28mm or wider, so the corners are all black. I was shooting w/ a 24-105mm w/ a circular polarizer and 10-stop nd filter screwed on top of each other. While doing long exposure, is a circular polarizer even necessary? Thanks for the advice!


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## mackguyver (Feb 2, 2015)

bereninga said:


> I'm playing around with some long exposures and wondered if circular polarizers should be used with an nd filter. At 24mm, I get really bad vignetting. It looked like the filter rings were actually visible in the shots of 28mm or wider, so the corners are all black. I was shooting w/ a 24-105mm w/ a circular polarizer and 10-stop nd filter screwed on top of each other. While doing long exposure, is a circular polarizer even necessary? Thanks for the advice!


I use a CPL and ND filter together quite a bit, but both of the filters are thin models. You only need to use the CPL for two reasons:
1. You need to cut glare/reflections from reflective surfaces and/or darken the sky
2. Your ND filter isn't strong enough to slow the shutter speed to what you want

I usually use the CPL for #1, but try to avoid #2 by having stronger ND filters. Also, you can always go a bit wide and crop.


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## TomF (Feb 19, 2015)

A note on polarization, two polarizers result in what is called cross polarization, which causes an increase in reduction of light transmission. This technique is used by geologists in optical mineralogy to determine internal structure as different minerals naturally polarize light transmitted through a thin section.

The net result is two polarizers can cause near opaqueness when properly oriented to each other. Someone, don't remember who, make a double adjustable polar. What I would recommend is a larger polar with a step up ring.

Good luck! Tom


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## TexPhoto (Feb 22, 2015)

Using a 10-stop nd filter, i can't imagine a polarizing filter is needed. But look at your exposure you used and see if you could have simply used a smaller aperture, or lower ISO.


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## sagittariansrock (Feb 22, 2015)

TomF said:


> A note on polarization, two polarizers result in what is called cross polarization, which causes an increase in reduction of light transmission. This technique is used by geologists in optical mineralogy to determine internal structure as different minerals naturally polarize light transmitted through a thin section.
> 
> The net result is two polarizers can cause near opaqueness when properly oriented to each other.  Someone, don't remember who, make a double adjustable polar. What I would recommend is a larger polar with a step up ring.
> 
> Good luck! Tom



I believe that is the principle employed in all variable neutral density filters.


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## Zeidora (Feb 23, 2015)

sagittariansrock said:


> TomF said:
> 
> 
> > A note on polarization, two polarizers result in what is called cross polarization, which causes an increase in reduction of light transmission. This technique is used by geologists in optical mineralogy to determine internal structure as different minerals naturally polarize light transmitted through a thin section.
> ...



Precisely. 
You can also use dual/cross polarizers to eliminate reflections from metal objects. But then you put one polarizer on light source, the other (aka analyzer) on lens. Just did that with dual gooseneck LED cold-light source on a stereomicroscope shooting harpsichord strings. Works beautifully. There, too, light intensity is strongly reduced; don't recall how many f-stops, but it was a bunch.

Polarizer plus ND-grads also work well. I use a Lee set-up for that.


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