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Old Oct 6, 2007, 3:52 PM   #1
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Using my new Sony Alpha 100 I tried to take a few pics using my Minolta Maxxum 70-210 zoom lens with a circular polarizer. The camera was unable to focus. Tried manual focus, and was unable to fucus manually in the view finder either.... I found that quite strange.

The 70-210 with filter focuses fine (AF or manual) with my Maxxum 700si 35mm body.
The 70-210 without filter focuses fine (AF or manual) with the a100.
The filter on the sony 18-70 zoom (that came with a100) seems to focus fine but pics reveal things out of focus at 18mm zoom.

Would anyone know what is going on here? Why is the polarizer messing up the dSLR?

Lens Specs:
Minolta Maxxum AF Zoom 70-210 1:3.5 (22) - 4.5
Sony 3.5-5.6/18-70 Zoom (came with a100)

I'll post up some test pics later tonight (have no access on this PC). I can also do a few experiments if people tell me what they would want me to do if it will help.

Thanks!
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Old Oct 6, 2007, 9:09 PM   #2
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I'd try a different polarizer (and make sure it's circular because a linear polarizer can cause problems with some Autofocus Sensors).

It could also be a problem due to light loss (you can lose around 2 stops of light with some polarizers) making it difficult for the camera to focus correctly.

The manual focus problem sounds odd. But, a cheap polarizer can cause optical issues and I do see some threads with problems like this reported elsewhere. See this one for an example:

http://photography-on-the.net/forum/.../t-103008.html


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Old Oct 6, 2007, 9:10 PM   #3
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I posted some quick test pics tohelp explain the problem:
http://picasaweb.google.com/jgajew/P...erFocusProblem

( I know the pics suck, but I only had a few mins to spare. Maybe I can take better ones tomorrow.)

Pics shot in auto-mode with focus set on the lens in the picture (did not change any setting to compensate for the filter).

As you can see, the lens that came with the camera (18-70) is quite bad @ 18mm zoom. Likewise, the 70-210mm is bad at 210mm zoom. Both lenses seem ok at 70mm zoom but still not a sharp as without the filter...

Why is the dSLR having issues when my older 35mm SLR had no issues?

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Old Oct 6, 2007, 9:21 PM   #4
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JimC wrote:
Quote:
I'd try a different polarizer (and make sure it's circular because a linear polarizer can cause problems with some Autofocus Sensors).
I'm postive I have a Circular filter - states so right on it.

JimC wrote:
Quote:
It could also be a problem due to light loss (you can lose around 2 stops of light with some polarizers) making it difficult for the camera to focus correctly.

The manual focus problem sounds odd. But, a cheap polarizer can cause optical issues and I do see some threads with problems like this reported elsewhere. See this one for an example:

http://photography-on-the.net/forum/.../t-103008.html
Hmmm... I suppose it could be a cheap filter. It was purchased at a chain photography store in Canada. The brand is Black's (http://www.blackphoto.com/). It cost just under $100Can ~10 years ago. How can I tell if it is cheap? Had no issues with it on my 35mm SLR with same lens.

Thanks for the link - I'll check it out. I posted here as I tried googling the problem and could not find anything...
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Old Oct 6, 2007, 9:33 PM   #5
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Well, if the filter is the cause of the issues and it's not just the light loss, my guess would be poor optical quality causing it. Being unable to manually focus either sounds pretty odd though. It won't work, regardless of how the filter is rotated?

As for good versus bad, you see all sorts of opinions on the subject. B+W filters are generally regarded as being very good.

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Old Oct 6, 2007, 9:41 PM   #6
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JimC wrote:
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Being unable to manually focus either sounds pretty odd though.
Thanks for the tips.

As for the manual focus, I can't focus because I can't get it sharp through the view finder. If it is not sharp throught the view finder, I know the pic will be out of focus. Maybe it is the artifacts of a cheap filter as suggested earlier....

I'll take some pics again tomorrow and eliminate a few things. I'll use a tripod, use the same ISO settting and aparture/shutter speed. I'll also note all the settings for better analysis.

Lastly, as suggested in the other post, I'll hit a camera store and see if a more quality filter makes any diff...

Thanks.
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Old Oct 6, 2007, 9:56 PM   #7
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It's hard to tell from those photos what's happening. But, it appears to my eyes that you had a flare and ghosting problem from the bright reflections, even in some of the images with no filter, and that in itself could be causing AF problems.

Taking more with more controlled settings sounds like a good idea.
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Old Oct 7, 2007, 1:26 AM   #8
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I had the exact same problem on my beercan with a CPL on it, the CPL is a generic brand, but the problem was gone after I replaced it with a Hoya Digital Pro1 CPL.
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Old Oct 7, 2007, 11:06 AM   #9
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Will try a different filter and report back.

In the meantime, I uploaded 4 new pics for those that are interested using the 70-210 zoom (did not bother with the other lens). All pics are uploaded using largest file in Picasa, so do zoom in to see the detail (it takes a few secs though). http://picasaweb.google.com/jgajew/P...erFocusProblem

The new 4 pics were taken:
- using tripod
- using aparture priority @ F8
- ISO 100
- automatic white balance
- Spot fucusing aimed at the peak of the roof (top of the triangle; center of picture) using AF
- the shutter speed is noted in the captions (but was 1/500 /w no polarizer, 1/200 /w polarizer for both 210 and 70mm zooms)

(and no glare this time
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Old Oct 10, 2007, 7:37 PM   #10
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For those that said it was the polarizer...they were 100% correct.

Went to the local Camera Shop store and tried a few different filters.

Took some sample shots at the store with the following settings:
-Aparture priority @ f4
-iso400
-70-210 lens @ 210 zoom
-focus was set at the middle of the letter H

Pics at: http://picasaweb.google.com/jgajew/P...erFocusProblem

I tried the following options:
- no filter
- my original black's polarizer
- a $35 quantaray polarizer (something like this: http://www.ritzcamera.com/product/241667401msk.htm?bct=t13031003%3Bcidigital-cameras-and-accessories%3Bcifilters%3Bcifilters-round)
- a $55 quantaray digital polarizer - apparently made for digital cameras (http://www.ritzcamera.com/product/241668334msk.htm?bct=t13031003%3Bcidigital-cameras-and-accessories%3Bcifilters%3Bcifilters-round)

Guy at the strore could not tell me what the difference between the two quantray filters was.

Results were that the quantaray was better than my original black's polarizer but not quite there. The quantaray digital was the same as having no filter (as far as focusing is concerned)

So, my problem is solved but now I'm curious as to why this is so? What do they do different in the polarizer filters for digital SLR's? And what is so special about the AF systems in dSLR compared to normal 35mm SLR's that a polarizer is able to have focusing effects on.
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