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.

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).