Just that split second decision to get the shot.
The XT-1 I played with in the shop seemed slower than what I was hoping for though my expectations are high coming from a 1d4.
I had a chance to play with the 50-140 and it was really struggling to lock focus quickly and completely failed at times, which was surprising and disappointing. This was just in the shop, not the best light but not exactly bad. I'm going to try the combo again elsewhere, hopefully with the new firmware as I think there must have been something strange going on that time.
It worked rather well though with the standard lens, forget the focal range but it was a F2.8-4 I think medium fl zoom.