Hi, I'm not going to spend a lot of time trying to convince you, but I think you are mainly being mislead by the B&H numbers. My guess as to what "3 (+2 stops)" means is this: +3 stops for daylight, or +2 stops for tungsten light.
Originally Posted by mporter012
Hoya's own information (p 47 of their catalog) says the filter factor is 8X (3 f-stops). They also say, "The precise filter factor is determined by considering the film type and specific light source."
The reality is that you should be getting your filter factors from film data sheets. A sharp cut filter like this simply does not have a filter factor on its own. The filter factor is based on a combination of the film's spectral sensitivity and the light source. And it assumes a neutral item in the scene; this is the basis of the corrective effect of the filter factor.
I would personally start with the presumption that Hoya's "25A" is roughly the same as a Wratten #25. (I would somehow double check this.) Then, if you look at some Kodak data sheets, you'd find that Tri-X filter factors should be (roughly) 8X for daylight, and 5X for tungsten light. T-max is similar, except only 4X for tungsten. If you look at an obsolete film, such as Tech-Pan, you'd find factors of 3X for daylight and 2X for tungsten light. From this, it should be obvious that metering through the filter is a bad method - the meter doesn't know if your film is more like Tech-Pan or Tri-X.
The Ilford site isn't loading for me tonight, so I can't say what's there, but it's worth checking if you shoot Ilford films. Same for other brands.
As a few other people have mentioned, the exact colors present in the scene are affected differently. So this is something to keep in mind. Likewise for any oddball light sources, such as energy-saving fluorescents, LEDs, etc. So a first roll should maybe be considered partially as a test. Good luck.
both Bills are giving excellent advice here.
Originally Posted by Mr Bill
You might be onto something. This is where having a wealth of different answers can be confusing, I saw Hoya's and B+H contradiction and didn't know what to make of it.
Film type, light source, meter type, how you meter, what you want. All play a part in black and white filter factors.
Film type: Depends on the film you are playing with. Some differences are significant and might be several stops (Ortho/IR). Other films (Technical/Tabular/Traditional) differences are subtle but might lead factors to vary about a stop .
Lighting: (Tungsten/Daylight/Open Shade). Could be a stop just as you pointed out Mr Bill.
Meter type: Have you seen how super-responsive a selenium meter (Master II) is to Tungsten. This is a documented reason why manufacturers used to recommend lower EI in Tungsten... With tungsten less blue, less actinic... And with meter jumping wildly to the light bulb... Old-timers had two problems going opposite directions - so something had to be done. They recommended lower EI in Tungsten. Now we have better meters.
How you meter: Simply does your setup or habit make you meter through the filter or do you meter without filter?
What you want: If you want detail in shadows then "Hutchings" factors help you obtain that.