R2, exactly which model of Tamron 1.4x TC are you referring to,
perhaps this? My application would be 70D, 1.4 TC, 300mm f/4 IS. Thank you.
Hi Bugbait,
Autofocus performance can vary with just the smallest of changes to a TC's parameters. For example only certain Kenko revisions would work with certain combinations of lens and body (and we used to tell them apart by the color of the dots on the TC!

).
Tamron has also made several different TCs (and revisions) over the years too. The picture of the TC in the link you provided at Amazon is not the same as my TC.
However the picture of the Tamron TC on B&H's website is the same as mine...
http://www.bhphotovideo.com/c/product/53109-REG/Tamron_AF14C700_1_4x_Teleconverter_for_Canon.html
But the model number is different than mine (MFR #
AF14C-700). Interestingly enough this is the same model number as in the Amazon link, just with a different (likely updated) picture. Maybe B&H just didn't update their image. Happens.
What's written on mine is...
TAMRON-F AF TELE-CONVERTER 1.4X C-AF1 MC4 JAPAN
I bought mine about 8 or 10 years ago.
I haven't used it with my 300 f/4L IS on the 70D yet though, funny thing. I'm so used to automatically grabbing the 400 for birds. Or using the 300 bare when I need the extra stop or the close focusing.
I would think that this combo should do quite well for you though. AF with the TC does slow somewhat, and IQ takes a hit of course. The Canon TC would give you a little better IQ I would think (less LOCA).
One thing I've noticed is that the 300 works much better on my 70D than it did on any of the other bodies I've shot with it. It was so bad on my 30D that I almost sold it. It wasn't that it needed MFA, but rather that its AF was so indecisive.
OTOH last weekend shooting my niece's play in very low light the 300 was just killer. AF was 100% spot on, literally. I can hardly wait to get out and shoot some more baseball with it. Come on spring!
Anyways, I hope this info helps. Good luck to you,
R2
--
Good judgment comes from experience.
Experience comes from bad judgment.
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