Gero - If I had one -big- complaint with the 90mm Summicron Pre-ASPH, it was the various color fringing in the bokeh. Sometimes it would be okay, other times is was everywhere. Here is example -
Here's a 100% crop from the red edge / arrow location -
That is LoCa aberrations. Nothing horrible, but it's there. Just about tele lens I've used has it (Canon, Leica, Zeiss, Mamiya, Hasselblad...) The problem with this CA is that with the right colors, it can really contrast and pop out. Like on Zeiss lenses the yellow-green hues contrast sharply with blue oceans, so a neon outline of sorts can appear in the bokeh along high contrast edges. It's a pet peeve and drives me bonkers. Alot of times I'll fix it in Photoshop, but sometimes it's just not possible to fix because of a complex backgrounds with alot of colors.
Anyway, the 90mm Summicron Pre-ASPH had this more-so than my other lenses. I tried a couple 90 'Crons and they all had it. Even the 90 APO had it (tried two of those) - that really surprised me. I thought APO's were supposed to eliminate this for of CA. The 75 APO does a pretty good job of controlling it.
For most people what I'm talking about is extreme bit picking, and I get that. But all the same, it bugs the crap out me and has led to extreme spending to find lenses that manage this form of CA better than others. Generally, I don't think Zeiss and Canon worry too much about the CA in the bokeh. Leica generally seems to have less, so I'm guessing their optical engineers try minimize it.
If Peter said the 90mm was just for landscapes and such, I probably wouldn't suggest the 90mm Summicron Pre-ASPH. For portraits it does a really nice job, especially if the backgrounds are fairly clear of potential CA garbage. Here's an F2 shot that may have some CA in it, but the scene completely masks it -
And a typical portrait -
The last two shots were taken with the M8 (if that matters). As a portrait lens, I think the 90mm Summicron Pre-ASPH is a great choice. I haven't tried the 90mm Elmarit, but that just arrived this morning