Quote:
Originally Posted by postcardcv
I've tested a 1.4x tc (actually tried both Kenko and Sigma) on the 170-500 with a 350D - neither gave AF and there was an obvious drop in IQ. Personally I wouldn't recommend using a tc with this lens.
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They wouldn't give AF - the Canon is f5.6 at the long end and the Sigma f6.3 IIRC. Add a stop for a 1.4x TC and you're outside the AF spec of the camera.
I don't know which make/model TC would give the best IQ, but I can tell you that the Canon one won't fit.