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Thanks guys. It was fun to meet up with Eric and some Toronto area photographers. I should note some of the images, like the fifth and last one, were cropped a fair amount.
Phil: I like the 21 Lux a lot, with certain caveats. Even with the 21/1.8 now available from Voigtlander, I still think it's a pretty unique lens if you're looking for both light gathering ability at this focal length and maximum subject/background separation. The Voigtlander is comparatively sharp wide open and the background blur difference is not considerable in the center of the frame. Where these lenses differ is in field curvature. Leica has apparently put a lot of effort into attempting to flatten the plane of focus as much as possible (as seems to be the case with all/most of their modern lenses, Luxes in particular). In reality it's somewhat wavy but the center and edges of the frame are about in the same plane. For example, this image from the series above:
The focus is on the Ricoh GR in the center, yet Eric's face is also in focus, as is the X100 in the hand of the photographer on the left. The CV has more typical, traditional field curvature where the focus towards the edges gradually falls behind the point of focus in the center. If shot on the CV, the two on each side may have been somewhat softer. What this also means is background blur/separation towards the frame edges and corners decreases with the CV vs. the Lux. Is this important in practical use? Probably not so much. But at this focal length where everything can quickly seem to be in focus, sometimes having that much more separation can be a subjective benefit. This point and the extra 2/3 stop of light gathering vs. the CV is why I still have the Lux. Shooting the wedding on the weekend there were a number of times when f/1.4 was very useful with the M9. But, shooting an MM or M240, which each have a 1-2 stop high ISO advantage over the M9, and maybe it's no longer such an important consideration.
The caveats of the Lux, IMO, are that it's really best shot at f/1.4-2.0, or stopped down f/8 or more, and its 70cm MFD. It has a nice, smooth rendering wide open for appealing subject/background separation, but stopped down around 2.8-5.6, it develops an odd mid-zone nervous quality where sharpness never seems to be quite right. It makes nailing focus on subjects placed in the rule of thirds quite difficult, and for infinity scenes, the mid-zone area will have a odd quality to the focus. At comparable f-stops, the CV and most other RF 21s I've tried don't seem to share this quality. This, and the high micro sharpness/contrast, are the major reasons I also own the 21 SEM, which is about the most technically perfect 21mm I've ever used. From wide open you can put the subject anywhere in the scene and it will be sharp. For a 21mm lens, Leica's adherence to a 70cm MFD is a limitation. You simply will never be able to fill the frame with small objects, but then, it might also be good for people photos as distortion at near distances certainly becomes quite pronounced. For small object photography, the CV (and ZM21/2.8) allow RF-uncoupled focus to 50cm. At this distance, the 21/1.8 pretty much matches the background blur look of the Lux at 70cm.
My reason for the 21 Lux instead of the 24 Lux was I really like 21mm and if images are a bit loosely framed, can crop appropriately, as I did with some in the series above. Some of the images could have been done with a 28 or 35, but as the light waned, the difference between f/1.4 and f/2 became more significant and I didn't have the CV35/1.2 with me (in part because it's so large). From some discussion with a 24 Lux owner, it appears it has quite similar field curvature and sharpness behavior characteristics shared with the 21 Lux - the MTFs from both are quite similar.
I suppose it will boil down to personal preferences. If you find 24 is wide enough and you're often cropping somewhat, then the 21 might not suit you. If you do a lot of stopped down work and want good across the frame performance at any aperture, then the 21 SEM or 24/3.8 would be better choices. The CV 21/1.8 seems to be the all around 'compromise' lens, but with excellent image quality - fast enough for a lot of low light situations, similar subject/background separation to the Lux, stopped down performance competitive with the SEM or ZMs. But the 21 Lux I really like a lot for candid people photos where f/1.4 really helps them stand out from the background more than you'd expect from this field of view.
If I haven't already bored you with my summary above, a few months ago I wrote up a more detailed review between the 21 Lux, CV, SEM and ZM: http://www.ronscheffler.com/techtalk/?page_id=130
Also, I'm not sure the 21 Lux is more popular than the 24 Lux. I seem to see many more used 24s available and at least here on this thread, there have been several 24 Lux owners, while prior to Ryan's joining, I was the only one with the 21. My impression is for many, 21mm is an infrequently used focal length and spending the money on the Lux is therefore difficult to justify.
Here is something that caught my eye on the way to the meet-up with Eric last night:
M9 & 28 Cron, 21 Lux, 50 Lux ASPH, 28 Cron