Exposure correction for filters in B/W film

Steve Simmons in his book " Using the view camera " says that when we

have to use a filter for B/W film we must take the measures through

the filter (with a spotmeter) and then apply the filter factor. In

this way isn't it like we apply the filter factor two times? Do you

believe is this procedure correct? Any experiences? Thanks.

Obviousy there should be OR between the sentences, not AND. You are right: if you do both, you are applying the factor two times. That can't be correct.

If you are using strong filters like a deep yellow, orange, or red, reading through a meter will usually cause underexposure with most B&W films. Even though you can find exposure factors for a filter, they actually vary depending on the film. Check the film manufacturer web site for details on using filters with the specific film you are using. But even then, the effect on exposure can change depending on time of day, humidity, altitude, etc.

It takes some experimentation if you want to get the filtered exposure absolutely spot-on. The light meter's spectral sensitivity does not exactly match the film's and the film's spectral sensitivity may vary from one type to another. But I just use the mfr's recommended filter factors and it works well for me.

The filter factors are usually pretty good. Another thing to remember is that if you have a bright sunny day with lots of blue sky, and you use a red filter to darken the sky, the shadows of your subject are illuminated mostly by blue light and will likely be underexposed.