For what it's worth, I was using a 180mm Mamiya Secor C lens on an RB67 MF camera with mirror locked up on a heavy Gitzo tripod. The filters were multicoated B+W MRC just bought-right out of their shipping containers. I was comparing shots taken with and without the filter, but they were different scenes. Probably has to do with that rather than equipment or, I hope, operator error. I'll shoot some more and compare similar scenes, with and without filters. Thanks for all your help.