This won\'t help you, but I think the 21 Lux has fairly similar character to the 50 Lux ASPH when shot wide open. Both aren\'t super, super clinically sharp, instead they have that hint of \'Leica glow.\' As a result, I believe the 21 Lux has a certain smoothness to its rendering that you don\'t get from the SEM, which instead is insanely sharp with pretty high micro contrast. Thorsten Overgaard had an interesting summary of the 21 Lux in his M240 running review. Basically also thinks it\'s best used wide open.
The SEM is the one to go with if you simply want technical perfection (as close as one can currently find at 21mm for M-mount). It\'ll be great for landscapes, city scenes & all around use. It\'s also pretty small & takes the typical 46mm filters.
The Lux is really best shot wide open to take advantage of its wide open look, or stopped well down. As I mentioned in the write-up, between around 2.8-5.6 it has odd mid zone sharpness that can be distracting if there\'s a lot of detail in that area of a scene. Therefore, I use it wide open (or f/2), or stopped down to f/8 or smaller. Another PITA with it is the stupid Series VIII filter implementation. Difficult to find those, and you\'ll want a ND for sure if you\'re going to shoot it wide open in the sun. I think B+W still offers at least a couple options... and Ryan K in this thread, I believe, confirmed you can use certain 62mm filters in there too since they\'re narrow enough to fit into the hood which acts as the filter retaining ring. If you\'re thinking about the Lux, then the CV might fit the bill without blowing your budget.
I guess it really depends on what you want out of the lens...
As for the 18 SEM... from what I\'ve read about it, it is somewhat weaker into the corners than the 21 SEM and will need to be stopped down to bring them up. Though not having used it personally, I\'m not sure how \'bad\' the corners really are, since everyone seems to have different interpretations of this.
There are at least a couple guys who\'ve posted here with the 18: Bijltje (Robert) and Bobu (Boris). Might be worth sending each a PM? Boris also had the 21 SEM (he\'s currently out of the Leica system) and should be able to give you a good idea about each. Another option is the ZM18, which is highly regarded, though kind of a fat lens. At least it uses \'normal\' filters. I believe the 18 SEM needs some sort of adapter for ~77mm filters.
Joe and Michael here both have/had the ZM21/2.8 and seem to like it. As I already stated, for me, I\'d go with the SEM over the ZM. You just never have to think twice about the SEM\'s performance.
From a walk in the woods:
Was messing around with B&W conversion options (channel mixer) in LR4 for the first and third, to amp up tonality/contrast differences in each.
M9 & 50 Lux ASPH for first and third, Nikkor 10.5cm/2.5 LTM for the second.
