[stoppingdown]

So, let's suppose I did this:

1. put the camera on a tripod and make sure everything is stable
   
2. autofocused on the center (a far subject, let's say 500m) and took a shot
   
3. switched to manual focus (and took another shot for a consistency check)
   
4. rotated the camera so the subject is placed at left and right border, and took two more shots
   

The border shots are in general less sharp than the center, as usual. Let's forget defects for now and suppose I'm fine with the results.

 

Now let's suppose I re-took the border shots, this time by reactivating autofocus, and autofocused on them by moving the autofocus cursor. In these shots, the focused border is sharper, but the center is worse than before.

 

Is this a normal behaviour, right? Is it due to field curvature? I have other questions, but first I'd like to understand whether I'm totally off track or not.