Today I've had the oppertunity to try the new 200-400/4 IS with the 1.4x converter 
I've never tried any longer lens than the 70-200/2.8 II IS, so it was quite amazing to have the oppertunity to try this one out. The place had quite bad lightning aswell so I had to use ISO 12.800 on the 5d3, but the images looked very nice.
I really like the fact that I could switch on the 1.4 so easily and the lens felt very "light" (as far as I could tell).. but very well built!
I've attached a picture of me trying it out with the 5d3 here. Have some more pictures on my phone. Have to bring my own 5d3 tomorrow to try it some more!
I've never tried any longer lens than the 70-200/2.8 II IS, so it was quite amazing to have the oppertunity to try this one out. The place had quite bad lightning aswell so I had to use ISO 12.800 on the 5d3, but the images looked very nice.
I really like the fact that I could switch on the 1.4 so easily and the lens felt very "light" (as far as I could tell).. but very well built!
I've attached a picture of me trying it out with the 5d3 here. Have some more pictures on my phone. Have to bring my own 5d3 tomorrow to try it some more!