In the interesting article just posted on the CV15/4.5 for use on Leica's M8, I see the statement that it doesn't matter which of the three possible LTM to M mounts you use. That's because each of the three brings up a different set of framing outlines in the M8's viewfinder: 28/90, 24/35, or 50/75 mm (actual, x1.33 for effective focal length). And none of these is right for the CV15. But if Leica makes its internal firmware vignetting corrections available through the upcoming release 1.10 which may include a menu entry to select the lens installed, or if you mark the black lines needed on the lens mount manually and use the present firmware, it is necessary in some cases to also have the frame lines set to correspond to what the Leica lens you are impersonating would display. For Leica's 21mm lens, this is the 28/90 combination. Since the firmware seems to be evolving rapidly, this warning may not be needed at some future date. But the vignetting correction is quite effective, and is applied before the DNG raw file is written, so that there is no loss of accuracy in the resulting image.