## Filter factors with 25A, ND, Polarizer

Hey all, this is my first post here and I was told to come here because you were very knowledgable regarding this stuff.

In a little over a week I'm going to be headed out to the Grand Tetons and Yellowstone for a week of camping and photography. For the trip, I bought a Hoya 25A filter to help enhance the look of the sky in my shots. While looking at the Kodak site, I found that Tri-X has a filter factor of 8 (3 stops) for that particular filter. It looks to be the same for Tmax films, and 2 stops for the Pan-X film. Has anyone here verified this? Also...how does the filter affect the meter usually? (ie - it might only meter that I need 1 stop more of light, but in reality I'd have to set the exposure compensation to +2 to make up for the total difference of 3 stops). I guess this may be something I'd have to play around with. I'll have to get real good at doing exposure math in my head :-)

I also have a circular polarizer that I may use in addition to the 25A. The polarizer I believe adds 1.5-2 stops to the exposure time, correct? Would the filter additions be additive? (ie - would it be 2 + 3 = 5 stops, or 2 x 3 = 6 stops?) I'm guessing additive but I just wanted to verify :-)

Also, looking through the the Tri-X and Tmax 100 technical publications, it seems to say that the filter factor for using a polarizer is 2.5 (1 2/3 stops). Is this in ADDITION to the effect that it'll already have on my meter or will this exposure change pretty much line up with my metered value?

My goodness...if I used my polarizer + 25A + ND 3x, I'd have about 8 stops exposure difference (and maybe some vignetting if I'm lucky, lol).

Thanks a bunch!