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| p.2 #6 · 5D Mark III - 41 AF points work at f/8.0! |
It is a mixed bag, it didn't work inside and only so-so outdoor . Inside, the AF hunted and hunted and it finally gave up . Outside, it tried and nailed the AF on distinct edges with good contrast and couldn't do that on less contrasty targets. Between you and me and the fence post, we can forget about using that combo for birding, especially in the AI Servo mode. However, you can stack two 1.4X TCs, one Canon and one generic, like a Kenko, and you have to tape the pin of the Kenko if it is a reporting TC and the stacked TCs will work well with the 500mm of yours. I had that setup a few years back and I liked it better than the 2X Mark II TC. Maybe that's the route that you want to take.
Have you actually tried the stacked converters with the 500 and the 5D Mark III, or are you just assuming that it will work like it did on other bodies? I'd be interested in confirmation that it will actually lock AF reasonably well and that it doesn't just frequently hunt similar to what you described for the 2x.