Circular polarizer filters...quite different from traditional polarizers...as variable ND filter??

Started Jan 10, 2013 | Discussions thread
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Peter Patricelli
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Circular polarizer filters...quite different from traditional polarizers...as variable ND filter??
Jan 10, 2013

I go back about 55 years with polarizing filters...first in sunglasses and then in film photography, and I was quite familiar with using the 90 degree "blackening test" to confirm that both sunglasses or lens filters were polarizing.

Then, when AF became the rage....it became necessary to buy circular polarizing filters for the AF to work correctly....and what exactly the difference was and why was never clear to me...but they worked about the same and I never inquired further.

Then there was a discussion here about the use of ND filters in doing times lapses, and I found VARIABLE ND filters for sale...apparently using two polarizing filters variably angled...but they were a significant cost....and I already had PF's on every single lens I own.

Sooooowhy not stack two and angle/dial them to get the same ND effect?  I ordered a second 77mm Circ. PF with the intent to stack on two different lenses AND have a backup.

When the new CPF arrived I screwed in onto the original, rotated them infront of my eye...and SAW NO DIFFERENCE!!!  I was going to send it back with an irate letter but decided to test all my current CPF's against each other and find they do NOT blacken out in the same fashion as the old, traditional PF's.  The color changes,and in some cases I get a color band, but their behavior, at least when stacked with another CPF, is is intrinsically VASTLY different than tqwo traditional PF's.

Can anyone explain WHY the difference?  If they can explain that then perhaps whether the variable ND filters being sold ....are they traditional or circular PF's.  In spite of my eyes not seeing a difference.....would the camera sensor??  Are the variables still AF compatible?

Thanks.

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Peter Patricelli www.flyfishingfotography.com

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