I traded in my Tamron 150-600 and bought a 100-400mm II. The Tamron at 600mm and f/8 on the 5DIII is as sharp at the centre as the 100-400 II at 560mm with a 1.4xTC. It is true that at f/6.3 the Tamron is a bit soft but at f/8 it is very sharp throughout its whole range and supersharp from 300-400mm.
I was too lazy to find it in the Canon material, but this is from the RF 1.4x TC product page on B&H. Lens compatibility in the description: RF 100-500mm f/4.5-7.1L IS USM (can only be used within 300-500mm range) RF 600mm f/11 IS STM; RF 800mm f/11 IS STM A good 1.4x teleconverter, when used with a Canon L 300mm f/2.8 or 300mm f/4, can outperform a good (but not great) 400mm telephoto such as the Sigma APO (not the APO macro version) lens. The 300 + 1.4 TC combination also roughly equals the performance of a 400mm Canon L f/5.6 lens. FF is of course more expensive, but the price differential
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hi Blake, i agree with most of your suggestions! my canon 1.4xIII extender and 600 f4.0II work quite good, it is the 2xIII + 600 II that turn pretty mushy! i don't have any issue with my rev. III extenders and 300 II. but like you have noted, i should send my extenders and 600 II to canon and have them calibrate the gear and hoping the issue go
420-700mm f/8-10. 600-1,000mm f/11-14. Canon RF 70-200mm f/2.8 IS USM. Not compatible. Not compatible. Canon RF 28-70mm F2 L USM. Not compatible. Not compatible. While it’s obviously disappointing that the 70-200mm won’t work at all with the teleconverters, and the 100-500mm only half works, both the 600mm and 800mm primes are fully . 452 54 273 249 73 216 138 427

canon extender ef 1.4 x ii compatibility