Some of us have played around stacking the 1.4xTC and 2xTCIII with an extension ring. They don't AF. I became intrigued on seeing the review of the Kenco Teleplus Pro 300 3XM DG, which is meant to be manual focus only 3xTC but it actually performs better than the Kenco 2xTC.
http://www.ephotozine.com/article/kenko-teleplus-teleconverters-2x-and-3x--review-14047
The 3xTC is no longer in production but some are being sold from Korea on eBay for about £800. A used one just came up on eBay for £50, and I decided it was worth a punt. It doesn't AF attached to a 300/2.8 II or the 100-400mm II but it focusses fine in liveview on the 7DII. It is very sharp, and I am going to use it on occasion with the 300/2.8 II plus 7DII, as an f/8.4 900mm! Can't wait for a clear night for a moon shot!
Edit. I should have written it autofocusses very well in liveview on the 7DII, the dual pixels make all the difference.
http://www.ephotozine.com/article/kenko-teleplus-teleconverters-2x-and-3x--review-14047
The 3xTC is no longer in production but some are being sold from Korea on eBay for about £800. A used one just came up on eBay for £50, and I decided it was worth a punt. It doesn't AF attached to a 300/2.8 II or the 100-400mm II but it focusses fine in liveview on the 7DII. It is very sharp, and I am going to use it on occasion with the 300/2.8 II plus 7DII, as an f/8.4 900mm! Can't wait for a clear night for a moon shot!
Edit. I should have written it autofocusses very well in liveview on the 7DII, the dual pixels make all the difference.
