never heard of a 120mm, sounds like a bit of an oddity.
Pentex made 42mm lens from 35mm to 150, for the most part, in 15mm increments, although from some reason they jumped to 105 rather than 100: 35, 50, 85, 105, 120 135, and 150. The 120 is somewhat rare, the 150 is easy to find. I find the 150 to be a better fit than the 180, I have a sologair 180 in 42mm as well, fast but just close to a 200. I dont think the 120 and 150 were carried over in the K lineup.
The 150 mm was not carried over, however the 120 mm was carried over in to the K lineup,
but not for long.
They certainly made an adapter for it, try this from eBay:
Adapter for CAMERA with lens M39 Leica to M42 Zenit
from Visit seller's Store: sovietmilitarystuff
I'm coming in late here, but to clarify - just to be confusing "M39" was also a thread designation used by some Soviet-era SLRs, specifically some Zenits. Despite what the sellers say, the M39 lens to M42 camera adapter listed on eBay is actually intended for the Zenit-mount M39 lenses so that they can mount on an M42 camera and focus properly. Although the thread diameter is the same as Leica screw-mount, if you try an LTM lens on an SLR you will only be able to get macro focussing with it due to the flange-to-film distance being different on a Leica (as with all rangefinders) than an SLR due to the SLR's mirror.
That said, I've noticed sometimes the Zenit M39 version of a Soviet lens is cheaper than its M42 version, so if that's the case for the lens you're looking for, and it's a manual lens anyway, if you have this adapter it might be worth your while.
Back to the main topic, my personal favourite M42 lens is the already-mentioned Jupiter-9 (85/2). The Takumar 35 is also quite good when I'm in the mood for something wider.
i can't wait to take a picture of my thumb with this beautiful camera.
- phirehouse, after buying a camera in the classifieds
I have discovered one of my M-42 lenses ....Super-Takumar 1:1.4/50... On my Canon FT b Gives outstanding clarity with a beautiful bokeh. I just posted a few on my Flickr photo page that is in my signature. I must say I was pleasantly surprised with the outcome. Nice small lens and built very solid. I will be looking for more Taks in the future
One lens to look out for is the outstanding Ultra Unitor Hi-D 135 1.8 with the 82mm filter diameter - this was also sold under the Weltblick brand. It's a huge piece of glass and makes an amazing portrait lens. I had a couple and now only have the one left which has a small patch of oil inside the rear element - I still can't work out how to disassemble it to clean this. They do pop up on ebay occasionally and I've seen a mint one with hood, caps and hard case sell for nearly $600 thoough usually they go for about $250.