Digital BW, The Print

I work for a good-sized commercial studio.  As such we tend to have 8 or 
so samples of our most used lenses.  With our Canon wide lenses, there 
is quite a variation in quality among wide angle samples.  In talking to 
a Canon tech at Canon Professional Services, he admitted that there's 
still quite a bit of sample variation among wide angle lenses, 
especially the zooms.  So if it's really important to you, make sure to 
test you lens when you buy it, if possible against a couple of other new 
lenses.  Buy the best one.

The new "Zeiss" lenses for Nikon and Canon are made by Cosina.  Ken 
Rockwell tested some of the lenses for Nikon and found them to be 
inferior to the Nikon lenses.  Now maybe his samples were bad, or maybe 
he screwed up the tests, but it is something to think about if you're 
considering buying them.