I believe the newer tubes if type E have electronic contacts and are 16E, 32E, and 56E. The tubes including the ones you mention can also be stacked. There is a chart which I don't have handy that will tell you the exposure compensation and other information about magnification, etc. It might be available on the Hasselblad web site or through a web search. I have a set of three above plus the 8. I think the 21 is considered useful for portrait shots. I can stack the 8 and 16 with my 150mm lens for about the same affect as a 21 with the 180.

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