Chromatic aberration is a different story. Color fringing is present and just into the visible range even on the 12mp cameras, especially in the corners. Vignetting is decent for such a wide angle, never hitting a stop in the corners (maybe two-thirds to three-quarters of a stop). Even setting f/2.8 pretty much removes the vignetting. Barrel distortion seems to check in at well less than a percent, which means I pretty much ignore it. However, it's pronounced enough that if you're shooting anything with straight lines you'll see it, so you may want to deal with it. (Remember, all the things in this paragraph tend to be easily correctable in software via post processing these days, so I tend to not get freaked out by anything but excessive problems in these three attributes. The 12mm is not excessive in any of these things.)

 

If there's any issue with the lens, it's flare. Light sources in the frame tend to produce visible flare. But without a hood, really strong side light also has a tendency to reduce overall contrast even if it doesn't produce a visible flare pattern. It's obvious enough that any engineer who put this lens on a camera should have seen it and recommended that the lens ship with a hood. Olympus ought to know better, especially on a US$800 lens. Unfortunately, we haven't gotten a lens hood with almost any m4/3 lens Olympus has produced. That means that it's an intentional tactic on their part to keep them as separate items for sale. Besides the loud chorus of boos that should produce, give them an additional chorus for not actually having the (how-hard-is-it-to-make?) lens hood available when the lens shipped. Finally, let's give them an additional raspberry for the (expected) high price of their hood. Funny thing is, the lens hood still tends to be out of stock at most places even at the high price, so it's obvious that users have discovered the same flare/contrast issues I have. "