... Unlike the oddball 16mm, the 21mm is a real, practical lens. Just like the rest of the lenses, you get autofocus and TTL metering for flash and ambient light, screw-in 55mm filters, and it's three stops faster than the 16mm.
... The only gotcha is that you have to compose through a separate finder slipped into the hot shoe. The finder is nicer than the finder of the Mamiya 7 43mm lens because its focus is fixed, but the Zeiss finder lacks a level.
The Zeiss 21mm has no visible distortion, although the separate finder sure does.(emphasis mine).
... The Zeiss 21mm is super sharp, even wide-open at f/2.8.
The Zeiss 21mm is so sharp that you have to pay attention to depth-of-field at f/2.8, even for distant subjects.
Unlike most 21mm SLR lenses that are soft at f/2.8, this Zeiss is ultra sharp. Because this Zeiss is so sharp, areas that are slightly out-of-focus at f/2.8 will be obviously out-of-focus compared to the in-focus areas.
Softer lenses seem to have deeper depths-of-field at f/2.8 because they lack a hard, sharp, in-focus core. With softer lenses, the out of focus area seems more acceptable because there is no significantly sharper in focus area to call it out. Cheaper lenses often are seen as having deeper depths-of-field because their overall softness misleads our eyes to perceive a sloppier, and thus deeper, area as "in-focus."
Because of this, I thought my first test shots of real subjects with the Zeiss 21mm were soft in some areas of the image at f/2.8, even though my tests on the mountain at infinity were sharp edge-to-edge at f/2.8. I first thought I might have a bad sample with some mechanical wobble. NO! What I was seeing was a lack of depth-of-field at f/2.8 because of the extreme sharpness of the narrow in-focus area. When focused at 20 feet (6m), objects that are a few feet (1m) closer or farther away are softer, due to limited depth of field at f/2.8.
I've never used a 21mm lens this sharp or this fast to be able to notice selective focus effects for distant subjects before!
Be sure to stop down if this is going to bother you. This Zeiss 21mm is good enough to show off sloppy focus technique.