Third Party Manufacturers / Re: Please advise which 1.4x teleconverter to buy: Kenko, Tamron, or Sigma« on: August 07, 2014, 08:04:32 AM »
For a 70-300mmL at 300mm with a Kenko 1.4X TC, its possible that your 6D will be manual focus only.
I just went on a photo safari in Tanzania with Canon 6D, 70-300L and the Kenko 1.4X. No problems with auto-focus. (mostly stationary targets - elephants don't move that quickly )
Before going I had thought about the Tamron 150-600 but I bought the 70-300L instead. But in Serengeti I could have used the extra reach.
I also brought the SX50 HS and found it a nice little camera- https://www.flickr.com/photos/larshaupthansen/14632565988/
The Kenko froze my 5DIII, as it did several for several of us. It turned out that it worked only when the AFMA was set to 0. Was your 6D calibrated for AFMA or was it left at zero?
The SX50 is very good for static subjects. The 100-400L or Tamron 150-600 are both great safari lenses.