I bought a Pro Master polarizer filter a few weeks ago and got the second test film back today. It is a 77mm used on an RB67 (127mm). I noticed the first time I tried it that turning it just didn't seem to darken up the sky like my polarizers that I use on 35mm. I don't like to crank the thing to the darkest spot anyway, I usually go about half way as I don't like these pictures you see on a beach with the sky looking like a storm is coming it. Anyway, about all I can say is that it does seperate the clouds a little, but no much. This is probably not the right forum for this question, but it is one of those questions that could go anywhere. I know that my manual focus camera does not need a circular but that was all they had. Is it the fact that it is a Pro Master filter and not a Hoya or Tiffen that is the problem or that it is not a linear? Thanks. Ric.