Teleconverters just don't work very well with zooms...

It really is just that simple. Sure, you can use a TC-14E on a 70-200 f2.8 or 200-400 f4, stop down at least a stop and get a respectable image. But the TC-17E and TC-20EIII? Forget about it. These higher powered TCs work pretty well on 300 and 400mm f2.8 supertelephotos or the 200 f2, but on my 500 f4 AF-S and 70-200 f2.8 VR the TC-17E gives inconsistent results - sometimes very good, sometimes not. On balance, it is probably better in most cases to shoot with the TC-14E and crop. And on the 300 f2.8 VR the TC-20III works much better close up than at a distance. End rant.

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Jim