Here are a couple of samples. I'm not actually sure wether these were wide open or even stopped down to 5.6.. anyway, not crazy sharp. I was actually expecting a bit more..
https://db.tt/BHgtpErK
https://db.tt/91ML8rqL
In the first photo focus was on the lady in the left and second on the lady in center. Straight out of camera, unaltered raw files to download and play around with...
DSC0095.ARW:
The light falloff at edges and corners, plus the focus zone, suggest an f/2 exposure. Compensating for that with the Lens Corrections panel in LR5.3, it looks like the plane of critical focus is about 1-2' closer than the woman seated on the bench, running slightly in front of the woman walking on the right. You can see how quickly the zone of focus drops off past the woman on the left ... normally you would expect that if she was the critical focus plane, there would be a bit more focus behind her than in front of her, but the reverse is true. However, there is still good detail in both of their clothes, and there's a little bit of the typical Summicron-R veiling 'glow' (coma, flare, whatever) from being wide open. You can see that in the test shots I provided by looking at the differences between the f/2, f/2.8 and f/4 1:1 clips.
DSC0097.ARW:
In this one, the light falloff looks much more typical of f/2.8-f/4, and the ISO has bumped up to 250 in similar light as the previous supporting that idea too. Again the critical focus plane is slightly in front of the central figure of the woman; it drops off a bit too quickly behind her. If you examine the ground carefully, there is evidence of some camera movement in the critical plane of focus (look at the ground, you'll see a little side-to-side blur on what should be stationary pebbles). There's still good detail in her dress, in her face and hands, but its lacking the crisp pop of a dead-on focus and vibrationless camera.
Judging from these two photos, the lens seems to be performing as expected. I'd suggest focus practice and work on holding the camera more steadily. I've seen camera movement blur even at 1/1000 second, so don't think just because the exposure is made at 1/400 second you're guaranteed to be steady.
G