OK, my turn. I know of 4 types of M42-PL adapters:
1) Official Pentax. Safe, tedious, not cheap. Everyone with at least one screwmount lens should own just one IMHO. I own one. Apply adapter to camera and it becomes a non-auto-aperture M42 camera, with however much lens (un)screwing is needed. Swapping lenses becomes tedious.
2) Fotodiox M/A type. This has an inner flange to hold down the M42 auto-aperture pin. I cannot find a source for this adapter, have never owned one, so I can't comment further.
3) Bower-type clone. Many of us with many screwmount lenses buy these in bulk for about US$5 each. Such clones have the notorious screw+spring assembly, and a dismounting tool. Remove the screw+spring or YOU WILL GO APESH!T! Then use a PK rear lens cap as a wrench to (un)tighten the adapter on the lens screwmount. I have had such adapters on some lenses for so long that I've almost forgotten they're not native PK. I've had no problems. Nervous folks might put a drop of contact cement on the threads for security, or even cut a slot or hole in the lens base for the PK lens-lock pin to engage.
The above are not 'flangeless' -- they all have a thin, narrow flange. For some lens adaptations, even that flange is too thick for infinity focus. So I've taken a couple Bower-type clones and ground away some of that thin flange. Tape a piece of rough sandpaper to a very hard flat surface like a kitchen tile. Use a rubber or wood block as a grip to press the adapter against the sandpaper. Do NOT grind away the flange entirely or it won't seat on the mount.
4) Wide-flange no-infinity-focus (NIF) adapter. Safe and cheap. I use these with macro tubes and bellows, for portraits and close-up work, for anything where infinity doesn't matter. If the adapter is anodized or painted, file or sand away down to bare metal in the appropriate place so it safely shorts the electrical contacts to allow catch-in-focus (CIF). I also use these on some longer (>100mm) lenses with very narrow bodies that won't normally short those contacts. An NIF adapter on a 200mm lens may drop far-focus to ~150m, but infinity may still be in focus when stopped down.
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Since I'm on a roll here, I'll also discuss M39 adapters. I note four types of lenses with 39mm mounts:
* M39, which have the same register (flange focal distance) as M42 and PK. These are almost entirely Russian AFAIK, made for Zenit SLRs; I've seen no others.
* LTM (Leica threadmount) which have a register much shorter than PK. These lenses CANNOT be used (without surgery) on Pentax dSLRs except for closeups.
* L39, common on enlarger lenses. These have no register. With the right extension (tubes and/or bellows) lenses longer than 60mm can reach infinity focus.
* A kind I've never encountered, don't know the name, not sure how they're used: 39mm thread, 0.75mm pitch. All the M39-LTM-L39 lenses have 1mm pitch.
OK, so what are the M39 adapters with 1mm pitch?
5) M39-M42 ring. This is a thin band threaded 39mm on the inside and 42mm on the outside, the band being about 6mm deep. I buy these in batch lots for about US$3 each shipped, for adapting M39 and L39 and some other lenses. With M39s, I've had no problems with infinity focus. I also non-destructively adapt Exakta-mount lenses. Glue an M39-M42 ring around the Exakta bayonets; screw on a ground-down Bower adapter I mentioned above. Voila! Infinity focus!
6) M39-PK adapter. Safe and pretty cheap. This a wide- and thick-flanged adapter, like the M42-PK NIF adapters I mentioned above, but thicker. I use these for close-up applications only.
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There's my overview. Which is 'best'? That depends on one's needs and budget. If you're doing close work, you can go even cheaper. Just get a PK body cap, the thick black generic sort that cost a buck or two, not the thin white one supplied by Pentax. Cut a round hole in the center of the body cap. Screw the lens in. Voila! Cheap and easy! This is also how I adapt thin lenses with odd mounts, like my Argus C3 glass. But that's another subject.
Last edited by RioRico; 01-26-2012 at 05:55 PM.