Quote Originally Posted by Thilo Schmid

I'm not sure what you expect from using a densitometer instead of a normal light meter for determining filter factors. Although a normal light meter might not exactly match the spectral sensitivity of your film, this is usually sufficient in practice. It is still photography and not rocket science!
If I were using a spot meter for my exposures, it would be a simple matter of metering through the filter. However when I am using incident metering I am not able to meter through the filter and must assign some exposure factor to the filter effects. I have long felt that filter factors are arbitrary in that the considerations involve not only the general density effect on light transmission but also the effect that a yellow, orange or red filter would have on the relative transmission of cyan in a sky for instance. I would like to know not only that a yellow filter has a 1 1/3 stop general density effect but also a 2 1/3 stop effect on a sky luminance for purposes of visualization at the time of exposure as an example. While the first effect (general density) is useful as far as a proper exposure, the knowledge of the 2 1/3 stop effect on a sky luminance is useful insofar as visualization is concerned.