I would be interested to know how good the Pentax 50mm 1.2 is over the 1.4. (I appreciate if you HAVE to shoot at 1.2 then you can't do that with a lens that only goes to 1.4!). The 1.2 is a very expensive lens relative to the 1.4 I have!
Guilty as charged!
I couldn't resist to the call of "the" 1.2 once I found one for a still decent price (£220), and last year I pulled the trigger on a very early K 50 mm f1.2, so far it's the oldest 1.2 known on pentax forum, on this 1975 brochure there is 120661, mine is 1200322.
Comment number 1 : you might get lost staring at that magnificent piece of glass for too long.
Comment number 2 : while most Pentax lenses are small, this one is too big for a LX, I prefer to use it on a K series body
Comment number 3 : the brochure boasts the fact this was in fact the first 50mm f1.2 (introduced in 1975, Nikon introduced his 50/1.2 in 1978 and Canon not before 1980. The fourth 50/1.2, the Fuji, arrived later) and indeed it was an achievement, the K mount was developed specifically for this lens.
In general, I can say the lens is built at top notch level, it's definitely better than the Canon FD 55mm f1.2 I have for my F1n (and half the weight), I don't know the performance of the Nikkor 55mm f1.2 but my understanding is that it wasn't that great, these two lenses were loaded with coma and flare wide open, the FD needs to be stopped down to f2.8 to become less muddy.
Here you can see a DOF study at f1.2 and f22 with the LX:
Sorry for the grain it's bad scanning...as you can see when it's focused properly it's relatively sharp wide open, but it's very, very hard to focus even with the ultrabright SA 23 screen, the DOF is so thin that might give the impression it doesn't resolute properly, just look at my face here:
On this hawk I focused at the leg but most of the subject is soft too:
Or this headstock:
At a certain distance separation becomes better:
Bokeh is of course great:
Low light performance also good:
Colors and resolutions are similar to the 1.4:
The lens is great for portraits:
All around:
In short I would describe it as a sort of 1.4 on steroids, if it does worth of it's up to you, thinking about the IQ and the price in comparison to a Leica Summilux I would say it's a bargain, if you like bokeh it's better than for instance the Canon FD 50 mm f1.2 IMO, here there's a Canon shot:
Here there's the Pentax: