Which lens is sharper at f/1.2!....That put me on the spot - Thanks Bart
...A quick off the cuff answer and I would say they are about the same...If you like your 40mm you won't be dissapointed in the 50mm.
No reason to own both, as they are not spaced far enough in focal lenth for most people...I purchased it to see if it would replace my 50mm Mitakon f/0.95 (Early days and I'm still on the fence and need to do some direct comparisons when I have some free time).
I suspect most people that buy either the Voigtlander 40mm and 50mm f/1.2 Noktons will not be shooting at "infinity" WO (I think night photographers would be sadly dissapointed with mapping the stars, as the extreme corners are going to be soft at f/1.2 infinity focused, although stopping down helps dramatically).
The market segment that I think this lens is being aimed at is street photographers (daytime and especially low light...ie. Vivek and Co.) where the rendering/bokeh subject isolation is probably going to be just as important as sharpness if not more so.
The 50mm F1.2 being the longer focal length naturaly gives you more OoF and subject isolation than the wider angled 40mm f1.2 at the same taking position (I personally prefer the 50mm WO seperation/isolation creamy rendering to the 40mm)...You need to get in closer with the 40mm to achieve similar (not the same) isolation of your subject matter.
Anyway enough faffing about, I'm just thinking how to best answer you
I just carried out some quick non scientific tests (Handheld no tripod) as the light keeps changing here (contrasty then flat) sunny with a lot of clouds and it's now dark
I won't upload the images taken at different distances WO but after some 50-100% pixel peeking...These are my thoughts
Both lenses on center WO look similiar in sharpness between 1.5m and infinity and are very acceptable (to me at least)....with about the 2-5m range focused distance showing a stronger sharper center to edges than what infinity does.
Below 1m focused distance the 50mm f1.2 I would say suffers the worst of the two lenses, as things are starting to get softer (glowy) and lower in contrast, where it would be best to add a diopter achromat to improve matters.
My final thoughts...The sharpness of the Voigtlander 65mm Macro Apo-Lanthar WO at f/2 is on a higher level than either of the 50mm and 40mm Noktons at f/2
Some links for you and others...
Fred Miranda - Inital impressions
https://www.fredmiranda.com/forum/topic/1561782/19#14829219
Fred Miranda - Improving sharpnes WO at closer distances
https://www.fredmiranda.com/forum/topic/1512530/36#14257362
Cheers Barry
Edit image added - 50mm Nokton @ f/8 nr infinity sharpness corner test downsized 4K pixel wide...Please click on Enlarge
In terms of sharpness it does a better job in my mind than my 50mm Summilux did at f/8
+ Fred Miranda is starting to run some comparison tests against other lenses
https://www.fredmiranda.com/forum/topic/1561782/20#14832246