I am the new proud owner of a Sony A900 with 2 Zeiss lenses (24-70, and 135 1.8). I ran some tests the other day, with the camera on a tripod, and the 135 lens mounted. I had set the Sony Image Stabilization Off since I was using the tripod, but neglected to do any sort of mirror lock-up. Looking at the files, shot at low shutter speeds - 1/30th sec, 1/ 45 sec, I noticed that they weren't that sharp. However the file i had shot at 1/200 sec was noticeably sharper. So I re-tested the next day, using mirror lock-up, and immediately noticed the improvement, all the files were sharper using the mirror lock-up.
I have read that having the battery grip on will dampen the mirror, and i have one on order anyway. But i am surprised by these findings. Does anyone have any insight for me on this?