I don't know of a web site with different "fudge factors" for use when metering through filters, but I have posted my method on this and other forums a time or two. A given set of factors will not work for all films anyway.
First a comment: I find it somehow comical that LF photographers make such a big deal about metering through filters when everyone else with TTL meters just slaps a filter on and goes shooting with their auto-exposure SLRs, etc.
I find it
more accurate to meter through the filter as long as a couple of things are taken into consideration.
First, probably most important and least known is that strong-colored filters affect the contrast of the negative by their color alone. The effect is dependent on the particular film being used. A #25 red filter increases contrast on good old Tri-X, but decreases it on T-Max 100.
Second, your meter's spectral response
will differ from the film you are using. Even the widely-touted Zone VI modified meters (of which I own two) do not match up to all B&W films. My unmodified Pentax spot meter only differs slightly from the modified version.
The above means that, especially for strong filters, metering through the filter can result in 1) a deviation from metered contrast, and 2) over- or underexposure. Overexposure is rarely a problem, since it will never be more than a stop and still results in a printable negative.
So, here is what you do (This assumes you have already calibrated your exposure and development.): Find a scene with lots of colors and a wide brightness range (use a Macbeth test chart for optimum accuracy). Make a control exposure based on a neutral color placed in a middle zone. This will be your exposure test area. Next, meter through each of your filters, using the same control area placed in the same zone and make an exposure for each. Note during this process how the corresponding and contrasting colors to the filter you are using change relationships in the scene. Make careful notes about where they fall (e.g. when metering through a red filter, note the values of red, green and blue. If you are really thorough, you could note the value of each color on the chart through each filter, but I find this unnecessary.).
Develop and print all the negatives and compare. You will find that the prints made from the "less aggressive" filters match the control area of the unfiltered print well and that only the tonal rendition of saturated colors has changed. Make sure these changes correspond to the notes you have made in the field. Factors for these filters will be small, if needed at all. Remember, it is a printable negative you want, not matching densities. Half a stop over or under requires no factor unless you are already on the edge of underexposing you film anyway.
The prints from the sharper-cut filters will likely exhibit a change in contrast and/or exposure. Make a visual assessment of these (e.g. "This print from the red filter looks too contrasty and underexposed.") After you have done this, estimate some "fudge factors" (e.g. "When shooting this film with a red filter, add 1 stop and reduce development by one Zone.") and go out and re-shoot. Develop and print, re-shooting as necessary till you have the factors you need. All the time, check to make sure that the highly-saturated values react as metered/predicted. You may have to add an extra factor for a color (e.g. "This film consistently renders green lighter than metered."). This latter is more something to consider when shooting that to compensate for.
Once you have done the above for a particular film, you are set. Put the results in your field notes or tape it to your meter and go shoot.
Best,
Doremus Scudder
www.DoremusScudder.com
