It's done in the beginning of the campaign. Check the intro script that comes with the first campaign mission (sharp soldiers, blurred background). I intend to open it up next weekend, but if you run out of patience that's where you'll find a solution.
ArmA cameras seems to be on a constant auto focus by default. If something passes the line of sight of the camera, it will adjust it's focus to the nearest target, closest to the central viewpoint of the lens - just as a real camera does.
I'm guessing here, but I would say that [50,1] from the script example in the comref refers to the distance where either the focus or the blur will be set to 100% (1 being 100%, since BIS is fond of scales ranging from 0 to 1 in almost all other commands, such as fadeSound and fadeMusic). The focus seems most likely, since the blur should appear at closer range as well as longer ranges than the selected focal point, while there is only one focus.
Will try this as soon as I have any testing opportunities. Supposedly, the changes doesn't occur until the camCommit X command is given.
EDIT: spelling