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I am confused about which filter factor to use. Should I use the filter manufacturers data or in the case of Kodak Plus X black & white film, Kodaks published filter factor data using wratten filters. It seems Kodaks filter factors are much greater than the manufacturers; in this case Hoya.
I never use the mfg's suggested filter factor. To begin with I meter through the filter. Then in the case of a dark yellow and light to med orange (#15, 16, and 21) I add one additional stop of exposure. In the case of red (#'s 25 and 29) I add two stops.
These figures were tested and developed before T-Max was available and may or may not work for the T grain films.
You should really test on your own to critically determine your own working practices.
steve simmons
The filter factors provided by manufacturers are intended to be an approximation - essentially, a starting point for your own testing. You may get some variation between films due to differences in the spectral response curve of the particular film. But, it's always a good idea to do your own testing, so you gain a better understanding of the filter's effect with your film and processing technique.
Note, too, that there are a couple of different methods for describing the filter factor - one as a multiplier (e.g. 2x, 3x, etc.) that needs to be translated into f-stops (2x = 1 stop, 4x = 2 stops), and the other as log factor (e.g. .3, .6, .9, etc.) that requires similar translation.
for FP4: #8: 2/3 stop, #12: 1 stop