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07-08-2013, 12:17 PM   #1
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misbehave's Avatar

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Out of Focus when using polarizer filter

I recently found out that my K30 with 18-135wr lens have hard time making the correct focus when zooming all the way to 135mm and with the circular polarzier filter on.
Is that because the filter I have is not of good quality? Or is it a common problem with this lens? Or do I have to adjust some options in the camera or lens when using such filter?
When I was using my Panasonic FZ30, I never had such focusing problem when adding a circular polarizer filter.

07-08-2013, 12:31 PM   #2
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There are linear and cicular porlarizing filters. The first ones can tamper with the phase detection AF system (PDAF) on DSLRs. Although I think this doesnīt affect pentax AF system, you should be ok with a circular pol (CPL). The quality of the filter isnīt relevant.

Also, at 135mm wide open aperture is f/5.6 which is the limit for the AF system. Adding the POL substracts 1.5 stops, leaving you with an equivalent aperture of f/9.5. Perhaps too dark for the AF system to work.

When using the FZ30, the AF system is contrast detection based (CDAF) so you wonīt have such a problem.
07-08-2013, 12:50 PM   #3
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Yeah, maybe the whole thing is too dark. Try focusing without the filter mounted and then just put it in front. after the focus is locked and before you take the photo.
Btw, do you have any other lenses that you could try the filter on?

Oh, and a bad filter can make the IQ of the photo so bad that it looks OoF, even though it isnt.

Last edited by Na Horuk; 07-08-2013 at 01:22 PM.
07-08-2013, 01:04 PM   #4
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Check to make sure it is a circular polarizer. Look through the side that threads to the lens at the reflection of the filter in a mirror. Now flip it over, looking through the end that is normally pointing away from the lens. The filter in the reflection should look almost black. If it looks the same either way it is a linear polarizer. Bargain filters usually turn out to not to be.

07-08-2013, 02:19 PM   #5
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See also:
https://www.pentaxforums.com/forums/pentax-lens-articles/65926-beware-cheap-c...polaizers.html
07-09-2013, 11:55 PM   #6
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misbehave's Avatar

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Original Poster
Thanks guy.

I've just did the mirror text with my CPL and turns out it is really a CPL and not a linear one.
I guess it is more like the case Carrlangas and Na Horuk mentioned, it was too dark.
I had this rare opportunity last week witnessing in a very close range of a Peregrine falcon trying to fight off a few other birds from stealing its prey (a rabbit).
I took tons of pics with that CPL on and now wish I hadn't took them with the filter on.
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