I thought I would share this comparison I did with a friend for a presentation I did for Sony and B&H.
This is how different focal lengths look with similar framing for portraits. The f/stop was f/5.6 on all of them. The portrait subject stayed in the same place, I moved progressively back to keep the framing similar as I used longer focal lengths. This was done with the 12-24/4 G, the 24-105/G and the 100-400 GM