Steve (or anyone else with filters that compromise image quality).

I am doing a study of filters on image quality and would like examples of known filters that compromise image quality. I have one polarizing filter that does that and I found the two sides of the filter are not parallel. So if anyone has such filters and would like to donate them to my study (which I will put on my non-commercial web site), please contact me off-thread. I would like to see if any of these filters have characteristics people could quickly assess before buying. You can find other articles on my web site regarding image quality.