I have an Olympus E-M5 and a couple of native lenses. I love it! But, like everyone, i hunger for more lenses. That hunger will not be satisfied by native lenses (mostly due to the impact it would have on my wallet), so i am looking at legacy glass. I currently have my eye on a particular lens, well, group of similar lenses, which are available on a number of different mounts: M42 screw, Praktica B, and Pentax K, and possibly others. Whichever i get, i am going to have to get an appropriate adaptor too. Now, once i have the adaptor, it will be that bit easier for me to use further legacy lenses with the same mount, because i can share the one mount between them. I'm unlikely to want to get a new mount with every lens, because i'm mostly focused on cheap but interesting lenses. So, in a way, in choosing this lens, i'm not just choosing a lens, i'm choosing a mount. It's like a miniature version of making the choice between Micro Four Thirds and the alternatives all over again! Therefore, i am wondering which mount is likely to be the best platform for future experimentation. Are there significant differences in the lens ranges available for M42, Pentax K, OM, etc? Which has the most different lenses? Which has the best lenses? Which has the cheapest lenses? Mostly, though: which one has the best availability of cheap but decent-quality lenses? I have no idea how to even start investigating this!