I just picked up a 16mm Extension tube and I plan to use it with my 150 for portraits, and my 60mm for general close up stuff. I saw the Calculator website, and got thoroughly confused. Can someone please explain this to me? Can I just add 0.5 stops to the exposure and call it a day, or would that be massively off? It'd probably work fine for negatives, but If I were shooting slide... Help!