100 f/2.8 L IS a remarkable Lens, sharp, fast focus on when you limit it, I recently used the D800 + Nikkor 105 VR Macro Lens, NO comparison, where the Nikon Lens was almost impossible to focus on Auto, the Canon 100 f/2.8 L IS on either the 5DMK3 or the 1Dx was simply superb, I've used this Lens now on the 5DMK2, 1DMK IV, 5DMK3 & 1Dx, works amazingly well on all these Cameras, if I had to choose a set up, it's likely the 1DMK IV due to 1.3 crop, allows me to get even closer than the FF allows.