Upload & Sell: Off
Geez, you guys are making me work hard. Okay, here is an example I concocted on the way home from work, where I tried to find not only a background which was in focus when it shouldn't be, but also walked around a bit trying different things in order to maximize it a bit. First an overall shot, with crop indicators:
The distance to the subject is about 8m. The top right is about 3x that, i.e. about 24m, and already there you can see that the texture of the building is starting to sharpen up more than in the centre, but the big "surprise" is when you move towards the top left corner, where one balcony is quite unsharp, and then one further away from the centre is much sharper.
Now if that isn't field curvature, I want more of whatever drugs I am taking.
The quality of the focus in the corners is not nearly as good as the quality of focus in the centre, that much is also clear. No matter what the distance, they just aren't tack sharp, so there is regular falloff involved too. I guess this is what Boris refers to, but the FC is definitively, very strongly, present, both in the corners and along the short edge.
Note that if the subject is at 6m, the FC is much weaker. The distance is very important. I haven't tried infinity.