Kaeseman Polarizer Overexposure

[laurent_martres1]

My new B+W Kaeseman polarizer used on an early Mamiya M645 with meter

prism yields grossly overexposed images (2 stops or more). It's a

mystery to me. Images shot without the filter are perfectly exposed.

Can anybody explain the phenomenon? Previous polarizing filter (Hoya

MC) provided accurate exposure.

[brian_hinther]

Is it a circular polarizer? Using a linear on a meter prism is about the only thing I can think of.

[laurent_martres1]

Forgot to mention that it is indeed a circular polarizer.

[rayn]

Interesting. My Tiffen Circular Polarizer greatly underexposes shots on my Elan II, about 1 to 2 stops. Exposure is fine without the filter. Now I meter manually and add +2 for the polarizer, works every time. I don't know why the polarizer fools the meter. It will be interesting to find out if anyone knows.

[robert_krasznai]

I also have a Tiffen CLP which I use with an Elan II, and EOS1n. I always use partial and spot metering respectively and the exposure is always spot on.This is with slide film.

[bobatkins]

I've never had a problem either.

 

Ray, you seem to be saying that if you meter a scene without the polarizer, then add the polarizer, your exposure doesn't change. Is

that correct? I suppose you can meter manually, then lock in the exposure, add two stops and add the filter, but that seems like a very hard way to do things.

 

There's no way a TTL metering system can ignore a polarizer. Even the circular/linear problem would rarely give rise to errors of more than about 1/2 stop, and a circular polarizers don't have a problem on any camera.

 

Something is very odd here!

[peter_olsson]

It sounds as if the meter compensates with 4 stops instead of

2 and therefore gives 2 stops of overexposure.

The only reason I can think of is if the Mamiya meter

itself has some kind of polarizing coating on the meter lens

or meter sensor. The combination of two polarizers would make

the light reaching the sensor in the meter become less. As you

know two combined polarizers

become almost black if the polarizers are "at 90 degrees" to each other. If you have this effect, less light would reach the

sensor and it would compensate too much. Therefore you would get overexposure.This doesn't

explain (not to me anyway) why the previous polarizing filter

didn't give any problems but perhaps the difference between

linear and circular polarizers accounts for that.

[laurent_martres1]

Very interesting reasoning, Peter, but to a weird problem there's got

to be a weird explanation. I should have mentioned one more detail:

the overexposed results happen with a new lens (a Mamiya 105-210 ULD

zoom) and a new polarizer (the Kaeseman in question). Prior to that, I

used to shoot with a combination of Mamiya 210 and Hoya polarizer and

never observed any advere effects. This would tend to invalidate the

explanation you proposed, but would instead shift suspicions onto the

zoom lens. Exposure is of course right on the money with that zoom and

without the Kaeseman.

[peter_wagemans]

Have you checked whether it is actually a circular polarizer, or

whether it was accidentally put in its mount with the back in front?

If you hold the polariser in front of a mirror and look through it so

that the light passes twice through the polariser, it should give a

darker image when the front is to the mirror but it should be (almost)

completely dark if the back of the filter (which is mounted onto the

lens) is to the mirror. A linear polariser doesn't show a difference

when flipped front to back and gives the same darker image in both

cases.

[laurent_martres1]

I finally got to the bottom of my overexposure problem and I'm sorry

to say that I wasted people's time in this thread, for which I

apologize.

 

I just got a couple rolls back. Images shot with this one particular

lens I bought recently from Keh are overexposed while all the others

are correctly exposed. So the lens was the culprit, not the filter.

 

After I bought the lens from Keh, I immeditaley tested it and got

perfectly exposed images. Then I installed the Kaeseman on it and

because it's a very neutral filter, I decided to leave it on the lens

permanently during my last trip. I did not imagine for a moment that

the lens could have become defective right after my first test.

 

The likely explanation now appears to be mechanical failure: somehow

the lens is not closing down to the correct f/stop and images are

taken at the lens' widest aperture of 4.5. What a bummer. Fortunately,

it looks to me like it should be a simple repair.

 

Thanks to all who tried to help.