The Lux:
- Pretty soft WO, gets very sharp in center starting at about 2.8, edges stay weak unless stopped way down. Like the 90/2 non-AA was designed as a portrait lens primarily where edges were not of that high a concern.
- Nice Mandler 'glow', like the 90/2, 35/2, 50/2 IIRC and a couple of others he designed
- Very good CA control - best I've seen in such a fast non-APO lens of roughly that FL
-Heavy
-Is well known for having modestly stiffer focus (like the older 90/2 non-APOs). Not crippling, but not as fast and smooth as more modern designs. Makes placement of focus point easy (vs say 35-70/4 which I also owned briefly which has a VERY fast focus - too fast, harder to nail focus point).
- Spendy
In comparison to the 80/1.9, I'd put the M645 at about 90% of the capabilities of the Lux - at about 1/6th the cost. Lacks that Mandler effect, but very good CA control, very sharp once you're down a stop or so, great bokeh, focuses smoother (a nice copy) and lighter.
I loved my Lux and preferred it over the 90/2 non-AA as the Lux gets as sharp at 2.8 in center as 90/2 does at 5.6, but after using the 80/1.9 I just couldn't justify a $1300 lens that had a $250 lens nipping so closely at it's heels and with both being manual stop-down, there wasn't enough of an advantage for me to the Lux to justify it.
Could you notice differences at 100%? Probably. In print, no - other than that 'glow' which I find quite nice. I have a print in my office from a 35/2 on Velvet Fine Art that actually has that glow effect and it's gorgeous, but the 35/2 costs a 1/3rd that of the Lux.
I've also heard some more objective Leica nuts (they do exist, though rare) that the Lux is not a whole lot sharper than say the Nikon 80/1.4. I'd owned the Nikon in a prior life and I'd agree - except for CA control. The Lux runs circles around the Nikon in that regard.
It was much like my 90/2 AA vs the CV 90 APO. Tested them on same scene at same apertures, looked at images at 100% and resolution was identical with the CV having less CA/fringing off a chrome item in the scene and the CV's bokeh was cleaner.
The 90/2AA (theory is due to the Asph design) is not as clean CA and bokeh wise as say the 100 APO. Looked at both lenses - one was $350, the other $1800 (and would need to go to Solms for months at a time for ANY CLA/repair work) and realized I could buy another nice fast lens to go along with the 90 CV to get the < 3.5 speed, keep the 90CV and have a large chunk of change left in my jeans vs keeping the 90AA. Away went the AA.
In short - yes they are very comparable. Identical, no. But close enough you'd need to do the cost/benefit in your head to justify the large $$ difference.
Unfortunately the Leitax mod option for Nikon and an increase in folks looking at Leica as they get fed up with (especially) Canon or Nikon options on high-res sensors (and a general increased interest in Alt glass) have pushed Leica prices to silly levels. A more viable option to the Lux if you prefer Leica color, etc would be the 90/2 non-AA. My pref would be for the more modern version (rarer) that looks identical to the 90/2AA - much smaller and smoother focusing.
The last design of the 90/2.8 is also a killer optic - almost a smaller, shorter non-APO 100 macro for 1/3rd the cost - but at a cost of 2 stops vs the Lux of course.
People also have to bear in mind that the cost advantage of the M645 glass is solely because the line has been dead for some years and until now, alt users have ignore them under the myth that MF glass sucks on 135. Everyone has been fixated on CY, Z_ and Leica. When new, a lot of the M645 lenses were VERY spendy. I do suspect prices will start increasing going fwd. Hell, I never thought I'd see a Mamiya MF thread on any board - let alone one that is so current every day -- and the more people mention a lens .....;>