"Really interested to see how this plus 1.4 Tc is vs the tamron "
Wicki - not sure if you are referring to the new Tamron zoom to 600mm against the 100-400 with Canon TC or this 100-400 with Canon TC vs. Tamron TC but here's my take on the latter: a bit disappointing. I spent the day with the annual Christmas Bird Count group here in socal and stumbled across a rare Lewis's Woodpecker. Stationary shots were wonderful no complaints at all (see sample Lewis's Woodpecker (20794)
by Anthony Gliozzo (Web Site is ocbirds.com)
, on Flickr however in flights shots were less than satisfying with respect to the camera and lens' focusing abilities . Despite it was on "3m to infinity" the camera was somewhat slow and struggled to focus on in flight bird shots against a gray sky. The combo offer only a max of 5 centered AF points. My friend was using also a 5D Mark III with a Tamron TC 1.4 and a Canon 300mm F/4 lens. All his inflight shots were quite good - his combo offered 9 AF points with the Tamron SP TC We set out cameras up the same - settings were AI Servo, Case 5, Spot Metering, manual mode at 1/1250 ISO 1000 Av=6.3 to perform this comparison. The 500 F/4 IS II has no issues - all points work with the same TC though I am now wondering if the F4 lens has something to do with limiting the AF points. Perhaps I can try my friends Tamron tomorrow and report back. ..cheers..Anthony