Originally Posted by
Ákos Lumnitzer
There is your biggest mistake. The Sigma lens works with the matching Sigma converter, however, that zoom is best not used with converters. I had a Canon EF 100-400/4.5-5.6L IS II USM zoom and sold that, as I did not like how it worked with a Canon TC. If you have to crop anything more than about 5-10% off your full frame shot at 600mm (960mm field of view with a 7D MkII), then you need to get closer. Cropping excessively (for me) is not the solution for good quality images. Sure, they look good on the web and social media, but that's all you get.
I purchased a new Sigma 500mm f/4 DG OS HSM Sport with its matching converters after seeing no difference in the field compared to results obtained with my old EF 500mm f/4L IS USM lens, which I have since sold off. In fact, I find its performance a touch better even wide open with both 1.4x and 2x TCs, believe it or not, with my 1Dx MkII. All this snobbery towards third-party lenses makes me laugh. I doubt I'll ever buy a Canon super telephoto again, they are too expensive and it just shows that Canon and Nikon have had photographers tight by the nuts for decades.