Please advise which 1.4x teleconverter to buy: Kenko, Tamron, or Sigma

[justaCanonuser]

I've decided to just use my 70-300L IS with my 6D and maybe later get a APS-C body for my secondary.
It would've been nice if Canon made their teleconverters compatible with the 70-300L IS (in all focal lengths) but they didn't and I've heard so many negative or "not-so-good" reviews on the 3rd party teleconverters.

Very wise decision. I have a Kenko 1.4x (Teleplus Pro300) that never made me happy, IQ didn't meet my idea of sharpness plus extremely slow AF performance with my 50D, later 7D and 5D3 with different tele lenses. Then I got Canon's Mark III TC's and it is the first time that I use in particular the 1.4x TC frequently, because it performs extremely well. I'd rather prefer to use no TC, as you decided now.

Btw when Canon's Mark III TCs hit the market, a German photozine did a lab review of 70-200/2.8 + 1.4/5x TC combos from different brands on full frame DSLRs (both the latest Mark II versions of Canon's and Nikon's 70-200/2.8 ). The result was that only the Canon combo delivered decent IQ, the Nikon and Sigma combos (the latter tested both on Canon and Nikon DSLR bodies) were no match.

 

[haupt]

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/

 

[Aichbus]

I'm going to throw in the Canon 1.4 mkii... get it used... get it cheap... and it works beautifully.

Canon extenders don't work with the 70-300 L, apart from that in my experience the Kenko's quality is on par with the Canon's (as far as the 1.4x version is concerned). it even adds less distortion. However, with the 70-300 L, I wouldn't use any teleconverter, for me the IQ with converters just isn't good enough, but that's debatable.

 

[AlanF]

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.