Quote Originally Posted by cliveh View Post
But is it not more likely that you could make an error in your calculations as opposed to reading the luminance at the focal plane? Or even do this to check you are correct?
not really. that's why you do exposure tests. Shoot film and see where you're at. with such bellows extensions it's pretty easy to be close to dead on in my experience (you can be out by several mm and it really doesn't make a difference). I'm far more likely to have variations in strobe power or development etc than be off in my measuring...