Photoclubalpha

Please help.
Immediately after I got a Sony a350 I went on a trip to the Southwest US, and since I love polarizing filters I shot almost everything using a Sigma 17-70 with that filter (a cheap digital Bower). The shots looked great through the viewfinder. The D-range optimizer is set to "advanced", and white balance to auto. Back home I was depressed to find that the a350 had over exposed everything shot with the polarizing filter. This never happened with my Olympus 5050zoom with a polarizing filter. Doing some experiments I found that photos were being overexposed by about 1.4 EV. I also discovered that removing the polarizing filter corrected the overexposure problem. The weird thing is that photos of clouds shot with the polarizing filter (with a -1.4EV compensation) look less polarized than shots with a plain UV filter (0.0EV)!

So is the problem:
1. The cheap Bower digital polarizing filter?
2. The Sigma 17-70 (I don't know how)?
3. The Sony exposure system is unable to use any polarizing filters?
4. The Sony can use these filters, but I have to compensate?
5. Something to do with the D-range optimizer?
6. The a350 Sony uses some "cloud magic" when it processes photos (duplicating the effect of a polarizing filter) so that a polarizing filter then is too much?

Thanks. Any suggestions are appreciated.

You say it was a cheap polarizer - was it a linear one or a circular one? The circular ones are more expensive but recommended for digital (though some report their linears are working fine).

My guess would go for the polarizer. I had a Bower polarizer myself, but got rid of it.

bossel wrote:You say it was a cheap polarizer - was it a linear one or a circular one? The circular ones are more expensive but recommended for digital (though some report their linears are working fine).

The filter was supposed to be circular, but who knows. Is there any way to tell?

deaton wrote:The filter was supposed to be circular, but who knows. Is there any way to tell?

Sure---and it's easy. Look through the filter into a mirror at the filter's mirror image. Flip the filter back and forth so you'd look through it once from the front side and then from the back side. With a linear polarizer, the filter's mirror image as seen through the filter won't change; the filter always will look grey. With a circular polarizer, the filter's mirror image will change from grey to pitch-black when flipping it.

When using a linear polarizer on a camera which needs circular polarizers then over-exposure would be the typical thing to happen. And the Sony A350 does need circular polarizers ... as all DSLR cameras do.

-- Olaf

01af wrote:

deaton wrote:The filter was supposed to be circular, but who knows. Is there any way to tell?

Sure---and it's easy. Look through the filter into a mirror at the filter's mirror image. Flip the filter back and forth so you'd look through it once from the front side and then from the back side. With a linear polarizer, the filter's mirror image as seen through the filter won't change; the filter always will look grey. With a circular polarizer, the filter's mirror image will change from grey to pitch-black when flipping it.

When using a linear polarizer on a camera which needs circular polarizers then over-exposure would be the typical thing to happen. And the Sony A350 does need circular polarizers ... as all DSLR cameras do.

-- Olaf

Your test works perfectly, and it did what you said. I even compared it with a Hoya polarizer which does indeed turn black. It is definitely a linear polarizer! What a piece of junk! I have bought cheap polarizers before and they always worked although I suspect that their coatings were not very good. I guess its off to the photo store to buy a name brand polarizer and to spend the money I suppose I should have spent to start with.

Thank you to everybody for your help!
Although I suspected it was somehow the polarizer, it did not occur to me that I had been sold a linear polarizer (mail order from an Amazon associated dealer).