This wide-angle lens features a rear-focusing floating optical system that gives a quick and precise autofocusing and removes close focus irregularities. It has a clicking sound it, which at first fooled me into believing that the lens was broken J. It is sharp in the center from f/2.8 but at f/4 to f/11 it really shines.
When reading about this lens, there is many mixed views on it. From what I can decode is that many people seems to confuse it with the 21mm and the old crappy 20mm MD lens. Let me state it for the last time… It is a good lens, with excellent qualities. I enjoy this lens while it is great for architecture, interiors and urban cape, also great for landscape. I have produced some wonderful images under complicated lighting conditions. Very high contract, fast, sharp and rich colour rendition. It is very easy to become addicted to this one,
To buy or to …
Looking in the hindsight, I am not sure I would have spent the 300 USD on this lens, I possibly would have spent the money on the 17-35 f/2.8 G instead, while it equally sharp at the same speeds, and even more usable.
A more technical run through can be found at Ken Rockwell