kidtexas
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carstenw wrote:
Zeiss lenses have really nice boke, but there is a lot more life left in it than in a Leica lens. The Leica boke looks smoother, in other words, but the Zeiss boke shows more of what is in the blurred region. This is exactly why Richard prefers Zeiss lenses and I prefer Leica lenses  A question of taste.
Some of this comes from spherical aberration and the correction of it. Lenses with under corrected spherical aberration tend to have bokeh highlights that are dim on the edges and stronger in the middle. This leads to really smooth bokeh - the Canon 50/1.2L and the Zeiss ZM 50/1.5 are examples of lenses that have this look. It can also lead to more pronounced focus shift. Over correcting spherical aberration can give rise to bokeh highlights with a bright edge (ring bokeh) and can lead to double-line bokeh in the background (nisen?). Perfect bokeh, often mentioned by brainiac, has smooth bokeh highlights with an even light distribution across the disk.
Part of my problem with these discussions is that most (all?) lens lines are not consistent in this matter. Also, different designs of the 'same' lens often have different properties. The Canon 35/1.4L and 50/1.2L are good examples of different behavior in the same line. The 35/1.4 tends to have busier bokeh than the 50/1.2.
The Leica M 50/1.4 pre-ASPH and the 50/1.4 ASPH are good examples of how a redesigned lens makes a big difference. The pre-ASPH is over corrected for spherical aberration, and this shows up in the bokeh at times. The ASPH, which for some reason has a reputation for 'harsh' bokeh, tends to have a smoother look since it has perfect bokeh, contrary to it's reputation.
Now, where was I going with all this? I don't know how it specifically applies to the CY lenses, the Z* lenses, Leica R lenses, Canon+Nikon lenses, etc., but I'm sure those lens lines have some variances, so making sweeping statements might be generalizing too much, especially with the lines that have been around for some time and gone through several generations, like Canon, Nikon, and Leica R. Design goals were different in the 60's and 70's (as well as being more limited) than they were from the 80's, 90's, and now. I'm more familiar with the Leica M line, and there are noticeable differences between the current line of lenses and the ones introduced 10 years ago. So I always think it's funny when someone tries out a 70's Leica R lens, and brands the whole Leica lineup with a given characteristic. M lenses can be very different from the R lineup, especially considering you can find a 50mm that was designed in the late 2000's or in the 1950's.
There does seem to be another lens characteristic (which may or may not be related, I don't know), which is how fast you go from the focal plane to a blurred background, at a given aperture. You often see this in lens comparisons. It's definitely a real effect, but I've never seen a demonstration of the fact that one lens line consistently does this faster or slower than another, only for specific lenses.
From what I've experienced, the Zeiss ZM line is pretty consistent across it's lenses. They might not be the *best* performers at a given focal length, but they are all remarkably good and consistent. I know this applies to there cinematography lenses as well, and I'm willing to bet it goes for their other lines too (Z*, CY, etc.).
Note - lenses with over-smooth background bokeh tend to have harsh foreground bokeh, and lenses with harsh background bokeh from being over corrected tend to have smooth foreground bokeh.
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Men are less sensitive to distortion 
I'm always amazed at how small M lenses are after developing my eye for size based on SLR lenses.