As an addendum to my previous post, here are some color images taken with the 2.1cm lens, taken in various places near Richland and Albany GA:
These are on various film stocks - either Portra 160 or 400, or Ektar 100.
The 2.1cm performs admirably on color as well in my opinion. There was only slight corner darkening in some of these, but nothing objectionable. Of course these are still negative films - slides might have more noticeable vignetting, I will have to see.
I'm loving this lens though. It's a real joy to use, despite the external viewfinder.