Mathia,
I am a HUGE fan of M42 lenses, especially my Helios 50mm/f2.0. As I shoot with a Sigma SD9, M42's are about my only cheap option for getting great lenses. [img]/ubbthreads/images/graemlins/grin.gif[/img] You're definitely right about the whole focus issue. It's takes a fair bit to really figure them out. I typically take three "frames" for every one "shot", completely spinning the barrel out all the way each time, then refocussing from scratch. Needless to say, I don't use these lenses for any level of action photography! It takes a bit of extra time, but I usually find that at least one or two of the three shots comes out sharp enough to blow away anything that could be done with a typical modern lens!
If you're serious about looking into screw mounts, get to know some of the lesser-know Eastern European manufacturers. They're typically the cheapest lenses you'll find out there, and their glass is usually superb. For instance, most Jupiter-9's are relabelled Zeiss manufactures!
Also look into other conversions, too. For instance, I _think_ Canons can also take Pentax K mounts with a couple of tiny, user-friendly modifications, and the Pentax PK's or 67's should be a cheap converter. Google it and see what you come up with. It's a great path to find!
-Jake
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