thanks for the quick response. I am still confused about that the other part of my question - I think I understand that the tube does not actually "recognize" a lens but I guess the numbers and how they vary between lenses for a range of apertures must reflect specific internal mechanics of each ...

I am not sure where to post bf tube questions - please advise on preferred location

Received my tube yesterday - very excited. Ran a stack yesterday and found the step distance to be very small - lovely image though. I could not distinguish between processing a 200 image stack one processed using ...