Greetings all you helpful folks. I have recently acquired a Schneider Symmar 5. 6/300 lens (born in 1973. It's a so-called convertible lens and thus by removin g the rear element it converts to a 12/500. Great, two lens for the price of on e! Since the f-stop is still read off the shutter I'm confused by the f12 thing when I remove the rear element. If I let my imagination go, I figure that the f12 thing is due to the bellows extension and that all I really need to do when using the 12/500 version is to read the actual f-stop off the shutter and then a pply a normal bellows extension correction factor (500^2/300^2 = 2.8). Unfortuna tely, the conversion from 5.6 to 12 is more than the 2.8 that I calculate (it's more like 4.1) Please explain how to adjust exposure when using the 12/500 lens. Many Thanks in advance.