I live in a screwmount world since I started photography, using M42 mount lens for my faithful Praktica, but I realized that I am perfectly ignorant of other lens mounting systems. Besides the Leica M39, all 35mm systems seem to have bayonet mounts, so I occasionally consider the thought of buying a bayonet-M42 adapter for some good glass, but I have no idea how many different systems there is. The only one I know for sure is the Pentax K-mount, because it superseded the M42 in Pentax bodies. So here's a trivia question for you all: considering just the basic needs of a) sticking the lens to a body, and b) closing the diaphragm at the moment of exposures, how many types of adapter am I likely to consider? In other words, How many common bayonet mounts is there out there, considering only reasonably available equipement, not odd stuff?