I have the 21/4 and shoot it on an M4-P with an adapter or IIIc. Absolutely love it. I've never had any problems with optical quality, but even if there were, it's tiny and a great lens for hyperfocaling in the daytime. F8, set focus to 2m, and anything you can't touch is in perfect focus all the time (even fast AF can't compete with that). Adjust for changing light with the shutter speed.

It was great when I was backpacking around Thailand/Cambodia.