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03-16-2010, 09:04 PM   #1
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Polarizing filter & AF

Hi,

I had linear polarizing filter mess an old 35 mm camera's auto focus. I just got my new K-x this week and don't want to that again. I had heard that DSLR could use either linear or circular filter.

Please point me in to the right filter.

Thanks,
NiteMove

03-16-2010, 09:21 PM   #2
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You need a circular polarizer.

Keep your linear for cameras with no TTL metering and AF, like some older film cameras, if you plan on having some of those?
03-16-2010, 10:33 PM   #3
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Any camera that uses a beam splitter in it's optical path polarizes the light going through the beam splitter to the component that is the target.
When you combine a linear polarizer with the beam splitter in the camera, you MAY get a loss of light to the target component because of cross polarization.
There is no Pentax DSLR that uses a beam splitter in the exposure metering, and in fact only one Pentax SLR of any kind used a beam splitter in the exposure system.
The AF is another story, but all that is happening is that in some situations, the AF sensor is getting less light than the nominal 1.5 stops that a polarizer normally costs.
In extreme cases, I expect that the sensor might be blinded, but I've suspect this will be rare.
For myself, I don't find that polarizers and AF lend themselves to being used at the same time, so I don't concern myself with circular polarizers.
I use good quality linear polarizers that cost me far less money than what a circular polarizer of similar quality would have cost.
As an aside, I believe you will find that auto focus systems have improved somewhat over the past few years and can work with lower light levels than in the past.
03-18-2010, 12:30 AM   #4
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I recently bought a 2nd hand 18-55 'kit' lens to take on holiday and because it has a 52mm filter threw in my old (25+ yo) polerizer (did they have circular version then?) and it worked fine, both metering and focusing.

03-18-2010, 03:31 AM   #5
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QuoteOriginally posted by Wheatfield Quote
Any camera that uses a beam splitter in it's optical path polarizes the light going through the beam splitter to the component that is the target.
When you combine a linear polarizer with the beam splitter in the camera, you MAY get a loss of light to the target component because of cross polarization.
There is no Pentax DSLR that uses a beam splitter in the exposure metering, and in fact only one Pentax SLR of any kind used a beam splitter in the exposure system.
The AF is another story, but all that is happening is that in some situations, the AF sensor is getting less light than the nominal 1.5 stops that a polarizer normally costs.
In extreme cases, I expect that the sensor might be blinded, but I've suspect this will be rare.
For myself, I don't find that polarizers and AF lend themselves to being used at the same time, so I don't concern myself with circular polarizers.
I use good quality linear polarizers that cost me far less money than what a circular polarizer of similar quality would have cost.
As an aside, I believe you will find that auto focus systems have improved somewhat over the past few years and can work with lower light levels than in the past.
Wheatfield provided all the answers. There have been several threads during the last months, tackling this issue in depth. So if you want to know more, do a Search here.

Polarizers are most often used under bright light (sunny day) to increase the sky's blue colour. Then you have enough light for AF to work fine with an old linear polarizer, too. Under dim light the AF may stop working with a linear pol - but who uses polarizers in dim conditions? That is very rare and then MF is always available.

Ben
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