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I posted this awhile ago in a few threads:
50 Lux ASPH:
A couple bonus Voigtlander:
These were shot on the GXR. WB based on the ZM image, brightness adjusted to match the white in the C of Canon. Of the various LR3 parameters, contrast was set to zero, black point at 5, clarity at zero. I guess this is more or less a bokeh test. There's hardly any color, so that won't really be a factor. Other than bokeh, the difference is really the degree of SA and how that affects contrast.
I don't have an apples to apples Leica-Zeiss 50mm comparison because I don't own a Leica 50/2. But I think these two offer some insight into the apparently different priorities of the two brands. This test actually disfavors the Leica considerably because it's weakest at minimum focusing distance, despite having a floating element design. It's surprisingly sharp wide open at infinity, though stopping down improves contrast/micro contrast. IMO, the 50 Lux ASPH, and other Lux lenses I own/tried, have some similarities. They tend to be dual-personality lenses. Wide open can be a bit gentle or slightly dreamy due to residual SA (see the softness long the top edge of "Canon"), yet still are very sharp. But the SA affects micro contrast. It's an interesting combination that results in a gentler rendering, which tends to be more pleasing for typical portraiture distances. Stopped down they're brutally sharp. Another goal of modern Leica lenses seems to be across frame performance with as flat a field of focus as possible, seemingly optimized for wide open too. I'd guess this is what aspherical elements help achieve? I see it particularly in the 21mm lenses. The 21 Lux has a much flatter plane than my ZM21/2.8. At nearer distance and equivalent apertures, this results in more background blur towards the image edge for the Lux.
Back to the 50s: the ZM50/2 is quite high in contrast, especially stopped down and I guess is fairly characteristic of Zeiss. That contrast/sharpness also holds at nearer distances, as can be seen in the greater 'pop' in the camera's textured finish. Whereas the 50 Lux ASPH has that wavy mid zone dip, which I think is caused by Leica pushing for an 'on average' flat plane of focus (edges recover to a similar point relative to the center), the Zeiss 50 has typical u-shaped field curvature. At infinity it seems to be better in the center and takes a while to catch up in the corners, meanwhile the Leica is really good from wide open. I never wanted to shoot infinity shots with the Zeiss wide open, but won't hesitate to do so with the Lux.
Then there is background rendering... The Lux ASPH, as you can see above, is very neutral. Even compared to the pre-ASPH version (see recent posts in the Leica M thread), there is quite a difference whereby OOF specular point sources tend not to have hard edges and there is less structure to background objects (the last aspect is not evident in these images above). The Zeiss does render harder edged circles. I feel the Zeiss is more consistent in its look through the aperture range with none of the dual-personality seen in the Luxes.
Then there are some of the newest Leica lenses, such as the 50AA, 21/3.4 and maybe the 24/3.8 too. These seem to lean into Zeiss territory, being very consistent in look across the aperture range, and also high in sharpness/contrast/micro contrast. I have a difficult time telling the difference between the 21/3.4 and the ZM21/2.8, in terms of contrast/color characteristics. Beyond f/5.6 it's also really difficult to see a resolution difference, but the Leica is superior wide open, with an extremely flat plane of focus. Carsten, it's interesting you mention transparency as a feature of the Zeiss lenses. I can recall this has come up numerous times in the Zeiss image thread. It's also a feeling I have about the 21/3.4. When I look at landscape shots I've done with it, it's a feeling of everything being sharp and well defined. I don't see telltale lens characteristics at play, other than perhaps pleasing contrast and color. So, my feeling is not only are some modern Leica lenses dual-personality, but so is the current Leica lens line. Some lenses will inject a certain type of character into the final image, whereas some seem to be quite neutral, very sharp, and seemingly transparent.