Strange performance of Zeiss Sonnar 135mm/ f2.8



I found the performance of my Sonnar 135mm f/2.8 is quite strange.

I tried some test with distant objects, and found that:
1. When wide opened at f/2.8, the resolution at the center is much lower than at off axis.
2. I stopped it down to f/4, the resolution at the center improved and close to off axis.

Most of the lens I used usually have inferior performance at off axis when wide open, my Sonnar 135mm f/2.8 give exactly the opposite. Is there any problem of my individual lens or it is a behavior of this Sonnar lens?

Here are the scanner of the test shots:

Please help.