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08-04-2012, 09:19 AM   #1
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circular polarizing filter seemed to have disabled AF

hello-

I wonder if anyone can explain what may have been happening to my K-5 last week when I was trying to take a photograph of a harbour while on holidays last week. I was using my 50 to 200 mm zoom with a circular polarizing filter on the end and looking out on a harbour which had large rock formations on either side and several small islands in the centre, to which I was aiming my camera. Regardless of the mode I was in, the camera simply refused to take the shot.I found though, that when I aimed the camera closer to me, that is at one of the rock walls to the side of the scene, the camera had no problem. Eventually I figured out that by turning off the autofocus I could get the shot I wanted. Other than the relatively large expanse of water, which I suspect is implicated in the problem, I cannot figure out why the camera was unwilling to take shot. I have taken pictures featuring water before, with and without the CP filter. I next discovered, that by taking off the polarizing filter everything returned to normal, even when using the autofocus.

I'm sure that if the camera goes on strike again under similar circumstances, I will remember what to do but I was wondering if anybody has experienced something similar and can tell me what was happening and why.

08-04-2012, 09:28 AM   #2
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A CPL filter can cost you up to 1.5 stops of light. At 200mm that would mean that your lens's effective aperture was F9.5-ish, at which you can't expect the AF system to work reliably, especially for landscapes.
08-04-2012, 03:22 PM   #3
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"Effective aperture"? A camera's AF works the same way the metering does, with the lens wide open. I don't know of a single 200mm lens whose widest aperture is f9.5. Unless you have manually stopped the lens down, the aperture used at the second the shot is taken has no affect on the AF.

I suspect that under the conditions you may have been trying to shoot under, that the C-Pl had probably reduced the apparent contrast of the things in the scene enough that the AF sensor was struggling. In those situations do what is commonly done. 1) Turn the camera to an orientation at least 45deg. (if the not the usual 90deg.) from what you are trying to shoot. 2) Lock focus and recompose.

Be mindful though that the effect of the C-Pl needs to be catered to when you turn the camera back to the desired orientation, because if you fiddle with it after recomposing that could throw the focus off again.
08-04-2012, 04:35 PM   #4
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I am no expert but a Pentax DA 50-200 is f5.6 at 200mm add one and a half stops is approx f9.5

08-07-2012, 06:45 AM   #5
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Thanks very much, all of you for the guidance. I'm sorry to take so long to reply. I haven't had internet access for a while and when I do, each time I visit the site I'm required to confirm my registration (though that is a different issue!
08-07-2012, 07:27 AM   #6
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QuoteOriginally posted by zekewhipper Quote
"Effective aperture"? A camera's AF works the same way the metering does, with the lens wide open. I don't know of a single 200mm lens whose widest aperture is f9.5. Unless you have manually stopped the lens down, the aperture used at the second the shot is taken has no affect on the AF.
The 50-200 has a max aperture of f5.6 at 200mm. Add 1.5 stops lost to the polarizer and you get what?
08-08-2012, 03:46 AM   #7
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QuoteOriginally posted by Adam Quote
A CPL filter can cost you up to 1.5 stops of light. At 200mm that would mean that your lens's effective aperture was F9.5-ish, at which you can't expect the AF system to work reliably, especially for landscapes.
But the circle of confusion doesn't change, so the light loss by itself should not be of any concern if this was in daylight.
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