veriwide, I do have a Luminar 3.5/25, too. It works well for 4x5 beyond 2:1. The only thing you have to know is that the numbers engraved on the aperture ring are not f-stops. They are exposure factors. On "15" you'll need 15x the exposure time of setting "1", independend of bellows factors. But you still have to determine the correct exposure for setting "1".