minus 4 filter takes 4 stops to correct for bellows

as stated a minus 4 filter on my rodenstock 135 took 4 stops to

correct for dark pictures using polaroid back. subject was at 36

inch distance on a table top shot to film plane(using flash).. would

this be due to the close up, or is this correctioin going to be

similar at infinity for my set up?. thanks dave..

Dave, you are dealing with extention factors from both the change in focal length of your prime lens due to the addition of the minus lens and from the close focusing distance to your subject. The addition of a minus lens will always require the application of an extention factor adjustment (also commonly called "bellows factor") since it alters the effective focal length of your prime lens. I make it a practice to carry a small tape measure with me to measure the bellows extention of my Cambo monorail and use a little "cheat sheet" glued to the lens board of each lens I use to arrive at the needed exposure adjustment.