Quote Originally Posted by Snapper
. . . As far as I can see there are 4 - an 8mm, 16mm, 32mm and 56mm. I'm not sure which to get
I believe there are also a couple of older, now discontinued models that fall in between those extensions. Also, I seem to recall there being a chart on the Hasselblad site that gives focus-distance ranges for each of the different extension tubes. Remember, you'll be using the extension tube in conjunction with the internal extension provided by the lens's focusing mechanism. Plus, the tubes (added: and/or the auto bellows) can be combined to provide greater extension. You just have to mount and dismount them in sequence so as not to disrupt the synchronization of the linkage.

The amount of extension provided by each of the tubes is relative, of course, to the lens with which it is being used.