[Sylvain]

Beyond DOF, don't overlook the f/2 will give you higher shutter speed. This would matter especially if you're into available light photography. At 12mm, the 12mm is probably 2 1/2 stops faster. The 9-18 is very small in travel mode but visibly longer when in use too.



Also, although PZ tests it to be in the same ballpark at 12mm, I am still a bit skeptic about the apparently large difference between center & borders measured here. Lenstip seems to confirm the trend but other sources (for instance : http://www.focus-numerique.com/test-1344/objectif-olympus-m_zuiko-12mm-f2-mires-3.html , http://www.slrgear.com/reviews/showproduct.php/product/1444/cat/14 , and admiringlight blog report on seemingly huge sample variation; they're perhaps all much less rigorous ?) don't mention such a large gap and rated the 12mm as very sharp from center to corners.



So it might be that a proper 12mm might be sensibly better than the 9-18 as far as sharpness goes?



ps.: comparing the focus numérique samples that are f/2 iso 200 to my 9-18 experience (my lens was the test sample of PZ), it looks to me Focus Numerique sample gives indeed quit egood corners : http://img1.focus-numerique.com/focus/articles/1344/olympus-m_zuiko-12mm-f2-exemple23.jpg