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12-31-2010, 12:09 AM   #1
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can a cpl go bad?

i have an older tiffen cpl that i used on my old quantaray telephoto lens on my old pentax film body, never a problem. now i have a new pentax da telephoto on my new digital body and i put my old cpl on but the pictures are just out of focus. also, i did a quick three picture test, fully polarized, least polarized, and no filter (on a tripod), the fully polarized is not sharp at all, the least polarized is very close to no filter. any ideas whats up?

12-31-2010, 12:53 AM   #2
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Are you sure it's not a linear polarizer?
12-31-2010, 01:28 AM   #3
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QuoteOriginally posted by kjacks Quote
i have an older tiffen cpl that i used on my old quantaray telephoto lens on my old pentax film body, never a problem. now i have a new pentax da telephoto on my new digital body and i put my old cpl on but the pictures are just out of focus. also, i did a quick three picture test, fully polarized, least polarized, and no filter (on a tripod), the fully polarized is not sharp at all, the least polarized is very close to no filter. any ideas whats up?
You need a circular polarizer, cause linear polariser are not compatible with the phase detection AF system; old polarizer are most of the time linear polarizer; you can solve this using live view focus. I have also read on polariser tests that some filters marked as "circular polariser" were linear.
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12-31-2010, 05:33 AM   #4
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QuoteOriginally posted by goubejp Quote
You need a circular polarizer, cause linear polariser are not compatible with the phase detection AF system; old polarizer are most of the time linear polarizer; you can solve this using live view focus. I have also read on polariser tests that some filters marked as "circular polariser" were linear.
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Linear pols work just as fine. All they might do is to reduce the available amount of light for the AF system - which would only be severe under dim lighting. And that is a condition where it is quite unlikely to use a polarizer anyway.

Just do a search on this topic. This has been discussed in depth several times this year and both, the theoretical explanation and the practical user experiences show, that Linear Pols are just as useable with Pentax DSLRs.

Ben

12-31-2010, 05:41 AM   #5
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QuoteOriginally posted by kjacks Quote
i have an older tiffen cpl that i used on my old quantaray telephoto lens on my old pentax film body, never a problem. now i have a new pentax da telephoto on my new digital body and i put my old cpl on but the pictures are just out of focus. also, i did a quick three picture test, fully polarized, least polarized, and no filter (on a tripod), the fully polarized is not sharp at all, the least polarized is very close to no filter. any ideas whats up?
Yes, polarizers can degrade over time. >Sandwhiched between two flat glasses is a polarizing foil, which can break-up due to moisture and air pollutants. This is one of the reasons, why B+W sells the Käsemann line of polarizers, which are sealed and offer better long time stabilty.

Ben
12-31-2010, 05:40 PM   #6
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thanks to all for your input. it gives unsharp images if i manually focus or auto focus so i'm pretty sure its gone bad.
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