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09-12-2009, 05:42 AM   #1
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Crossed stacked polarizers for ND..

Here's a quick experiment showing the effect of crossing stacked polarizers on exposure and tint (white balance constant). As you can see, quite a few stops exposure are added before the tinting effect becomes a problem.


The first in the series was with uncrossed polarizers which decreased the exposure about one stop. Depending on details one can likely get about 4 stops of variable "neutral density" filtering before tinting becomes a problem.

Dave Martin

PS the tinting effect is due to the transmission properties of the polarizing filters in the UV and IR.

09-12-2009, 03:11 PM   #2
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A very informative test. Thank you.
How much does it depends of the filters you use? Manufacturer, quality (price), linear/circular.
Did the colour changed if you rotated the "stationary" polarizer, or is it always blue.

I've red that cilcular polarizers are essentialy the same as linear except that they add another layer wich disturbs polarization in the filtered light exiting the polarizer.
I wonder if you could get such layer and place it in front of the filters (or reverse the front filter), so it wouldnt work as polarizer.
09-12-2009, 04:19 PM   #3
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QuoteOriginally posted by ytterbium Quote
A very informative test. Thank you.
How much does it depends of the filters you use? Manufacturer, quality (price), linear/circular.
Did the colour changed if you rotated the "stationary" polarizer, or is it always blue.

I've red that cilcular polarizers are essentialy the same as linear except that they add another layer wich disturbs polarization in the filtered light exiting the polarizer.
I wonder if you could get such layer and place it in front of the filters (or reverse the front filter), so it wouldnt work as polarizer.
I've tried it with a variety of filters & the results vary a little, but not enough to worry about. I've seen different tints with different polarizers, but this depends on both the polarizer and how much IR is present....crossed polarizers are usually transparent in the infrared.

With circular polarizers, the one farthest from the lens should be reversed; this will effectively de-polarize the incoming light. Reversing can be done permanently by removing the filter element, turning it over and remounting it. Or, get a reversing filter ring

Because such crossed filters are transparent in the infra-red, they provide a route to easy, variable, broad-band IR photography.

Here's a picture of a rose taken without crossed polarizers:

And with crossed polarizers:

Need help about: ND-filter made with two POL-filter: Pentax SLR Talk Forum: Digital Photography Review

The brightness of the leaves in the second photo is a result of the crossed polarizer's IR transmission; it is, in effect, an IR photo (almost all visible light is gone.)

Dave

PS The problem people have with trying this trick is that the inevitable tint that appears at the most extreme condition is upsetting; however,you can get quite a few stops of "neutral density filtering" before tints become a problem.
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