Many auto-focus systems do not perform when the effective maximum aperture is beyond f5.6. I do not recommend the use of a teleconverter on a lens with maximum aperture already at f5.6 such as Canon 100-400 f4-f5.6.
If a lens has maximum aperture f2.8 then both 1.4X (1 stop) and 2X (2 stops) can be used but many find acceptable or imperceivable degradation of quality using 1.4X but 2X it becomes more marked and its a debate within itself.
A teleconverter will exaggerate lens imperfections and should only be used on the front of the finest glass, there is no substitute for a 500 or 600mm lens.
How about a Sigma 50-500mm 'bigma' for greater reach only a little more cost than the best teleconvertors I am sure quality should be as good if not better and 100/400 with TC.
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