Quote: bdery, is your 21 mm really sharp (even wide open), fast, etc? what is its sweet spot--5.6 or 8? seems like the 15 mm got all the love, and the 21 isn't as highly rated as the 15.
It's amazingly sharp, I'd say. On the same level as my FA 50 macro, probably. Plus it has that "special something" that many people have called the "3D effect" what really pops out of the frame. It's a great lens, one that continues to amaze me.
If I'm not mistaken, the MTF-priority mode of my K20D (designed to use the lens at its sharpest setting if possible) tends to select f3.8, showing that the lens is made to be sharp wide open. My own informal tests suggest that when you close down near f11 it gets less sharp. It really is designed for wide apertures.
I cannot compare with the 15, and I'm sure that one is a great lens too.
Regarding AF speed, the 21 is again at the top of the class, about as fast as my fastest focusing lens, the F50 f1,7.
I'm not saying you should absolutely get that one, as I cannot compare it with the 15. I can only say that I love mine and will keep it in my bag at all times.