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12-15-2020, 11:54 PM   #1
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What happened to the polarizing filter?

Have a 82mm Vivitar linear pola filter that must be 25+ years old. It has been used sparingly on the Pentax 6x7 and 75 F4.5 wide-angle. Got it out the other day to use on the Pentax 645Z and HD 35mm D3.5 lens.

Acck! The filter has an oval about 5mm long that has lost its greyness. The oval is slightly tan tinted. At first thought it was something on the filter. Cleaning didn't help a bit. Then I realized the tan oval let's more light thru than the rest of the filter.

I always thought pola filters were grey because the polarizing film reduced some of the light, and that the fragile film was between glass? If so, what happened to the filter?

Thanks, gonna have to buy another,
barondla

12-16-2020, 02:34 AM - 1 Like   #2
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I'm not sure what happened to yours, but I can imagine that the plastic film layer can degrade over time as most plastics degrade over time. Then again, UV light is the most prominent cause for such degradation and the minimal use only would suggest that there shouldn't be too much degradation happening and the fact that it is localised doesn't make too much sense either.
12-16-2020, 03:11 AM - 2 Likes   #3
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If it’s old it might be degrading. It might have been exposed to heat.
12-16-2020, 03:12 AM - 2 Likes   #4
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If you do not have an expensive filter which has sealed edges (here in Germany called "according Käsemann") moisture will creep into the filter and degrade the polarizing layer. But this will usualy happen fromt the edges towards the center.
An completely uneducated guess from me would be, that he thin layers of glass of the filter got under preassure and the polarizing layer between them was destroyed this way.

However you are aware, that with a linear polarizing filter exposure measurement and phase AF will not work properly?
Thus one could say, you now have the unique opportunity to replace an inappropriate piece of gear by the latest, shiny new.

12-16-2020, 05:07 AM - 1 Like   #5
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Apart from beeing faulty, I think you will need a circular polarising filter to work properly with DSLRs. (I can't remember the technical reason for this, but it's something I read when buying a filter 5 years ago)
12-16-2020, 06:18 AM - 1 Like   #6
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Here is a great article on "Lens Filter"!
https://photographylife.com/lens-filters-explained

Cheers
12-16-2020, 08:44 AM   #7
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QuoteOriginally posted by SharkyCA Quote
Here is a great article on "Lens Filter"!
https://photographylife.com/lens-filters-explained

Cheers
Thanks for the link. This is a great source,

12-16-2020, 09:54 AM - 1 Like   #8
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QuoteOriginally posted by Papa_Joe Quote
If you do not have an expensive filter which has sealed edges (here in Germany called "according Käsemann") moisture will creep into the filter and degrade the polarizing layer. But this will usualy happen fromt the edges towards the center.
An completely uneducated guess from me would be, that he thin layers of glass of the filter got under preassure and the polarizing layer between them was destroyed this way.

However you are aware, that with a linear polarizing filter exposure measurement and phase AF will not work properly?
Thus one could say, you now have the unique opportunity to replace an inappropriate piece of gear by the latest, shiny new.
A linear polarizer can affect AF if there is sufficient cross polarization with the half mirror to blind the AF sensor. I'm pretty sure that Pentax DSLR cameras all meter off the focusing screen, and thusly are not affected by cross polarization. IIRC, the only Pentax that could have it's metering affected by a linear polarizer was the LX, as it's metering was done from inside the mirror box utilizing a half mirror, which is where the cross polarization, if any, takes place.
12-16-2020, 10:53 AM - 1 Like   #9
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Polarizing filters absorb light, they partially transform light into heat.
Consequently, their life is more or less short: more you use them, faster they deteriorate.
12-16-2020, 05:55 PM - 1 Like   #10
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Sounds like it's time for a new one. But make sure to get a circular polarizer filter.
12-16-2020, 06:00 PM - 2 Likes   #11
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Understanding Polarizing Filters - The American Society of Cinematographers

QuoteQuote:
The polarizing filter features a dichroic absorbing layer sandwiched between two layers of glass. The dichroic layer, sometimes called “pola foil,” is made from polyvinyl alcohol (PVA) plastic that is stretched during manufacturing, forcing the molecules in the plastic to line up in long parallel chains with tiny gaps between them — think of the bars of a jail cell, or a picket fence. These chains are invisible to the naked eye. This stretched polymer is then dipped in a solution of iodine, and the iodine molecules attach themselves to the polymer chains. The resulting structure allows the pola foil to absorb light waves that are parallel to the long chains while allowing waves that are perpendicular to pass through unaltered.
The PVA or iodine may have been unstable due to contamination or defective manufacture, glass could be separating. Your guess is as good as mine. I have a few Vivitar filters (non-polarizing) from the 80s that are separating.
12-16-2020, 07:50 PM   #12
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Thanks everyone. Apparently it has degraded. I've seen lots of old polarizing filters and never ran across this. Here is a picture of the filter. The linear pola didn't seem to cause the 645Z much trouble. I intended to buy a new filter before this happened. Was experimenting to see if the HD35 ( 28 eq FF) played well with a pola, was too wide, or possibly vignettes. All seems fine.

There are wire polarizers for machine imaging to tolerate heat. Fascinanting. Wonder if the image quality is better?

Thanks,
barondla
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Last edited by barondla; 12-21-2020 at 11:06 PM. Reason: Spelling
12-16-2020, 11:08 PM - 1 Like   #13
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Interesting, we're learning new things all along and everywhere. I can see what is the matter with the old filter. This seems an odd place for this degradation, also in light of what papa joe has written. The mystery endures but there is a reason to buy a new one, so all is not just bad.
12-19-2020, 07:22 AM - 1 Like   #14
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Makes me want to put a UV filter on my UV filter.
12-20-2020, 10:20 AM   #15
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QuoteOriginally posted by Wheatfield Quote
A linear polarizer can affect AF if there is sufficient cross polarization with the half mirror to blind the AF sensor. I'm pretty sure that Pentax DSLR cameras all meter off the focusing screen, and thusly are not affected by cross polarization. IIRC, the only Pentax that could have it's metering affected by a linear polarizer was the LX, as it's metering was done from inside the mirror box utilizing a half mirror, which is where the cross polarization, if any, takes place.
You are correct, it doesn't seem to affect the meter much, if at all, on the 645Z. Wouldn't a linear polarizer theoretically have better optical quality since it has 1 less optical element?

Thanks,
barondla

Still amazed that a filter wears out. Do other filters?
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