I recently got a Canon 5DmkII.
This weekend I was at the local high school football field playing with the camera and recording video. I set the camera down on the field, turned live view on, and pressed the focus button in order to focus on a soccer ball. In the live view, the depth of field was amazing. I was so excited to record. When I pressed the center button to record all of sudden the depth of field changed. It was not even close to what it looked like during in the live view before recording.
Why is this?