Quote Originally Posted by mhv
(snipped) ...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?
Let us take a step back and look at your requirements:

You have a collection of M42 screw mount lenses, and desire to get a newer camera with bayonet mount, for some purpose. You also like the idea of being able to fit your existing lenses to that camera and make them function as well.

Assuming that, you need a camera which can take an adapter that mates the M42 lens to its bayonet body. Brands of manual-focus cameras I can think of, off the top of my head, which allow that would be Canon Contax-Yashica, Konica, Minolta, Pentax-K, Praktica-B, and Rollei. These adapters fill in the gap between the lens and the camera, and retain the full focussing range of the fitted lens.

With the exception of the Rollei (which was designed to work with one model, the original SL35 which had stopped-down metering), none of these adapters can retain automatic diaphragm actuation: the M42 mechanism is triggered by a plunger pin which works parallel to the optical axis, but all others work in an axial direction. So the lens has to be used without any diaphragm automation whatsoever. Some of the adapters have the mechanism to force the lens to work as manual diaphragm, but most do not, requiring the lens to be switched to manual diaphragm completely. Forgetting to do so gives severe overexposure because no matter what setting you have on the aperture scale, it will simply refuse to stop down. And of course, for lenses without an auto-manual switch, those lenses would not be very practical.

The QBM mount by Rollei works in a different way, where the diaphragm actuator also works in a direction parallel to the optical axis. In other words, the adapter contains a simple relay mechanism which can indeed stop down the M42 lens fitted. But the companion camera with which it was introduced, the Rolleiflex SL35, had stopped-down metering, so it does not affect the functionality at all. With later models such as SL35M, SL35ME, SL35E and SL2000F and derivatives, they contain an extra coupling for full-aperture metering, so you can certainly do the metering with it, but you have to remember to stop the lens down manually when you are doing it.

Praktica provided the neatest arrangement where the adapter not only forces the lens to function as manual, but also contains a little box of tricks which electrically couples to the TTL metering system and tells it to meter at working aperture, effectively cutting off the aperture indexing and simulation system. I do not know if it is a good idea to make a start with the B-series now as Praktica has terminated the production of 35mm SLR cameras some years ago, and lenses are getting quite tricky to acquire.

There are also adapters which allow M42 lenses to fit other cameras but to retain the focussing range the adapter needs to contain a weak negative element which works like a weak tele-converter. Cameras allowing such adapters to be used include Nikon and Olympus. I do not see much point in starting a Nikon or Olympus system because the adapter introduces another element to the optical configuration, thus taking the edge off the lens performance.

Alternatively, you can also get a better camera body with M42 mount; I think Voigtlander's Bessaflex would be a very sound candidate if you want to follow this route.