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02-08-2011, 01:59 PM   #1
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K10D won't focus through polarizing filter

I tried a polarizing filter for the first time today on a K10D with a 55-300mm at max zoom, and autofocus would not work. Without the filter it worked fine. I tested on a some duck decoys floating 40-50 feet away, and at the sharp skyline of a distant hill on blue sky. Filter is a cheap one, I assume (Bower), that came with lens. Any thoughts?

02-08-2011, 02:07 PM   #2
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Interesting.

I've tried expensive polarizing filters and cheap polarizing filters, circular polarizing filters and linear polarizing filters with my K10D (I don't have the 55-300mm lens, however), I've never noticed any problem with autofocus.
02-08-2011, 02:40 PM   #3
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Perhaps the issue with extreme zoom, then, on this lens. You seem to have quite a selection to comment from. I'll play with other settings. Thanks.
02-08-2011, 03:03 PM   #4
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This refers to a Kx , but could it be your problem? Digital SLR | Pentax Support

" Q: Can polarizing filters be used with the K-x?

A: Yes. Circular type polarizing filters (C-PL/Circular PL) can be used. Non-circular polarizing filters can affect the exposure and autofocus and should not be used"




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02-08-2011, 03:26 PM   #5
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Seems like it may be related, but I do have a circular filter. Tried a few more shots and it seems the lens will autofocus fine from 55 to about 150mm, then can't. Some interaction between the optics of the lens and polarizing lines, perhaps.

---------- Post added 02-08-11 at 02:45 PM ----------

Found this elsewhere on the site. Apparently ran out of light at 150mm (sunny day, who would have thought). Looking into getting more photons (OEM, not eBay). Meanwhile this from Pentax, which I think applies here too:

"The autofocusing problem you are having with the K-r is probably being caused by the relative low speed of the DA 55-300mm lens combined with the filter factor introduced by a polarizing filter. ... With that lens I would suggest you try autofocusing without the polarizer and then attach the filter to the lens, or use manual focus only with your polarizing filter."
02-08-2011, 04:03 PM   #6
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Adding the light loss from the polarizer to max. f=5.8 @300mm might be enough to explain the problem. Also, in theory a linear polarizer could be a problem. The light passing through the main mirror to be reflected to the AF sensor by the secondary mirror is in fact polarized as if going through a linear polarizer; two linear polarizers in the light path to the AF sensor can dramatically reduce the amount of light getting to the AF sensor, if the axes are at about right angles. Circular polarizers prevent this by adding a quarter-wave plate "behind" the actual polarizer layer; this restores a mix of polarizations so that a fraction of light will pass the main mirror regardless of the angle of the polarizer layer of the CPL. (Or that is how I understand it currently )
02-08-2011, 05:27 PM   #7
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Thanks for the explanation. I thought something like that might be happening, but don't know optics.

I found this post elsewhere also on the site that explains that a circular polarizer may not in fact really be one, which can be determined by some simple tests. The one I'm using is. I'm going to continue to ry different settings and another lens and see what happens as I learn about al this.

https://www.pentaxforums.com/forums/pentax-lens-articles/65926-beware-cheap-c...polaizers.html
02-09-2011, 12:20 PM   #8
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Interesting - I've used a couple of different types/brands of both CPLs and PLs on my 55-300 with virtually no problems at all - used on a K200D, K10D and my K20D. However, the K10D did occasionally have focusing problems while the other 2 didn't, and not just with the 55-300.

I was never really impressed with the K10D - both the K200D and K20D always seemed to outperform it in regards to AF. My .02!

02-09-2011, 12:28 PM   #9
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I've used my Cokin CPL on my K10d without issue, mind you nothing i had it on would be f5.6 so i can't tell if that is the issue

I have a couple of old linear PL but the only lenses they fit are all MF
02-10-2011, 05:12 AM   #10
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Has the metering been affected as well?

To me, it sounds like the old chestnut of circular polarizing and linear polarizing issues.

Some cameras just don't like linear.
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