Here is my test with the ZM 18mm Distagon F4
http://www.flickr.com/photos/photonblast/11765829514/in/set-72157639417383524
You can see the field curvature on the A7. You get the corner sharp, the center is gone...
This is Wide open at F4. At F8 you will get flat field focusing.
Lower Left
Lower Center
Lower Right

http://www.flickr.com/photos/photonblast/11765829514/in/set-72157639417383524
You can see the field curvature on the A7. You get the corner sharp, the center is gone...


Lower Left

Lower Center

Lower Right