I'd have thought that a 2X converter on a zoom lens was really asking too much. I don't have a 2X converter, but I do have a 1.4X Canon TC, and I don't think the image quality of the converter & 100-400 is up to scratch. With the 500 prime, it's fine - very nearly as good as the bare 500, and better than the bare 100-400 at 400mm too. The differences are small, but they are there. If I was to try and quantify them, I'd say TC & 500 is one (very small) step down on the bare 500, and two of those same very small steps up above the 100-400.
It's probably worth mentioning that I generally shoot the 100-400 at f/6.7 (because I think the half stop improves the sharpness - might be imagination, but that's what I do), and do the same with the 1.4 & 500 combo, but I shoot the bare 500 wide open wherever possible.
Anyway, my feeling is that expecting ultimate sharpness out of a zoom lens (100-400 or 70-200) with a 2X converter on it is just going to lead to frustration. Too much glass in there.
But thankyou for raising this question. I've practically never used the 100-400 & 1.4 combination, only a few shots to try out when I first got it. So, just for fun, I'll have a crack at it again one day soon.
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