5D-III focus spot size
The focus sensor and image sensor have to be aligned precisely. I can tell you from experience on the 7D that dropping the camera can affect this alignment in a very negative way.
If you put the camera in AI Servo mode, spot focus, and keep the shutter half pressed as you pan from foreground object to background object, you can intuitively tell how the alignment is working and where focus takes effect.
|big gourd by rok urankar|
|crashing waves-1 by stickpointed|
from The Wild Rugged Coast
|'Red 18' by Private Custard|