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.
|Bianca Buitendag-7809 by vbuhay|
|Sunrise in Paradise by OB Foto|
from Booby Prize
|The Battle for the Lead by Photo Pete|
from Bicycle sports
|Cecelia's Eyes by Madeleine Hart|
from - Jenny from the Block - (Colors that pop + A white Border)