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Accessory talk Those important extra system components.

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Old 2nd November 2016
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Linear polarising filter help!



I'm playing with a linear polarising filter to reduce reflections of my car's dash in the windscreen when placed in front of my dash cam lens. There's no AF, so I'm advised linear will work OK, BUT... having taken the filter out of its mount I then dropped it on my desk and so I don't know which way around it was originally.

So... with a linear polariser, does it matter which surface faces the camera?

I'm aware that a CPL only works one way around, but can't find anything about linear filters?

Anyone?

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Old 2nd November 2016
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Re: Linear polarising filter help!

I think (but am willing to be corrected if wrong!) that a linear filter works either way round. The reason a circular doesn't is that it is a linear filter followed by an "un-polarising" layer called a quarter-wave retarder. If you use it the wrong way round the second layer scrambles the polarisation of the incoming light so the polarising layer has nothing to work with.

Easy way to check is with some reflections off a non-metallic surface like water or polished wood. Best is at about 30 to 45 degrees from the surface.

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Old 2nd November 2016
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Re: Linear polarising filter help!

John has it totally correct.
One point to add though is that the linear polariser doesn't stop AF.
It affects Phase AF but not contrast AF, I believe it can also mess up metering in some SLR cameras...
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Old 19th June 2017
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Re: Linear polarising filter help!

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Originally Posted by Petrochemist View Post
John has it totally correct.
One point to add though is that the linear polariser doesn't stop AF.
It affects Phase AF but not contrast AF, I believe it can also mess up metering in some SLR cameras...
Late to the party but......

Linear filters do mess up metering some cameras, including spot metering on the OM4Ti and OM2Sp as it happens.

The idea of the quarter-wave retarder 'scrambling' polarised light is to fool the camera's metering and focusing systems into thinking that the light is unfiltered.
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