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- Kelly Abernathy

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Polarizer Problem

Yesterday I had a problem with my autofocus locking into clear focus with my circular polarizer on. I shoot a D100, and had my 80-400 VR lens on. It seemed if I was zoomed all the way out at 80mm, the focus was clear. But if I was zoomed in at all, the focus was blurry. I was shooting handheld, but at shutters of around 1/800, so camera shake wasn't an issue. And when I took the polarizer off, there was no problem. I'm cleaning the camera and lens today, but I could see no visible dirt, dust or fingerprints on the lens or filter that might cause it to not lock on the scene clearly. I wasn't in a situation that I could have manually focused as I was shooting action shots. Here's a shot of a tree I took off to the side just to check if the autofocus would lock on. After this I just removed the filter so I could shoot the event. Any ideas? Thanks for the help! -K

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0 September 08, 2004

- Kelly Abernathy

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	Blurred at zoom with polarizer Kelly Abernathy

Here's the shot -K

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0 September 08, 2004

Jon Close

(a) I don't beleive that the 80-400 VR is par-focal (retains focus when zoomed). You have to zoom first, then focus.

(b) AF will become iffy in lower light levels. While the f/5.6 maximum aperture at the 400 setting is nominally wide enough to enable autofocus, when you use the polarizer you lose about 2 stops of light.

(c) You are using a circular polarizer, right? If by chance it is a linear polarizer, then it will confound the AF system.

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0 September 08, 2004

Anne House

"when you use the polarizer you lose about 2 stops of light"

Jon, Does this mean you could use a circular polarizing filter interchangeably(instead of) with a nuetral density filter? Just curious. Thanks.

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0 September 08, 2004

- Kelly Abernathy

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	Test-Clearer at 150mm with polarizer Kelly Abernathy

Hi Jon - I appreciate the info. You're always very helpful. You're right, I do have to zoom first then focus, which I did. It is a circular polarizer, Nikon brand. The shots were exposed okay (TTL metering), just the autofocus problem and I believe there was plenty of light with the polarizer. Settings were f5.6, 1/160, ISO 400 at 400mm. I'll attach a shot, same settings of f5.6, 1/160, ISO 400 but at a length of 150mm. It's clearer, what's the difference? Is light the issue even though it was metering okay with the TTL metering? I've used the polarizer quite a bit, and this is the first problem I've run into. I know I've used it frequently on my 18-35mm, but some too on the 80-400, although I don't remember if it was at the higher focal lengths or not. Any ideas? Thanks again -K

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0 September 08, 2004

Jon Close

Dunno.
Looking again at the first shot posted, I wonder if the blur is not that the shot isn't focused, but maybe camera shake? Any chance the VR was inadvertently switched off?

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0 September 08, 2004

- Kelly Abernathy

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Nope - it was on. And I could see through the viewfinder that the shot wasn't focusing. Any idea where I might start some research? Thanks -K

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0 September 08, 2004

Steven Chaitoff

Anne, the two filters aren't interchangeable. They do different things so you shouldn't use a PL filter where you need an ND one. It's like wearing socks on your hands if you don't have any mittens. It'll get the job done, but that's not what it is made for. Keep in mind a PL filter has tiny grooves that only go in one direction. So if you turn it, the reduction in light will vary, typically between 1.5 to 2.5 stops. Plus, common ND filters will reduce light evenly 2, 4 or even 8 stops or more, which is much more than a polarizing filter will do. Also, the main point of a PL is to reduce glare & reflections, which an ND filter won't do.

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0 September 08, 2004

Anne House

Thanks, I appreciate the response. I knew it was too good to hope, hehehe.

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0 September 08, 2004

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