I'm post this here because the M42 is so often called a Pentax thread mount and these lenses are often used with Pentax Spotmatics. I am trying to choose between three 200mm M42 lenses and I was wondering if you guys could help. Also, if you know anything about the actual manufacturers of these lenses, I'd be greatly appreciative. The first lens is a Vivitar 200mm F/3.5 Auto Telephoto #28610273. The second is a Soligor TELE-AUTO 200mm, F3.5 #17100327. The last is a lens marked AUTO-ALPA, Multi-Coated, For ALPA Swiss, 1:3.5 200mm. It's #2540049 and is made in Japan as is the other two. I've found a little bit out about the Vivitar, but not the other two. I'm looking at which is going to produce the sharpest, most contrasty images, etc. I'm also looking at which of these will be the most reliable. I use M42 lenses with a variety of cameras, a Zenit 122, Zenit ET, Pentax Spotmatic II, and the most recent of my acquisitions is my Voigtländer Bessaflex TM. Thanks in advance for any info you can give me.