Can anyone comment on the quality you would get with this Canon teleconverter? I've never owned or used one. I'm looking for an alternative to spending way too much cash on a 500 or 600mm fixed-focal length Canon lens, to shoot sports with. I just can't justify the cost and renting too much of a problem, also. I have a Canon 100-400mm zoom lens, that accepts the Canon 2X extender. This would give me a 400mm "reach," and yes, I realize I'd lose two stops of light but I wonder how the quality holds up? Autofocus works well? And while we're on the subject, has anyone used the YongNuo teleconverter? https://www.amazon.com/YONGNUO-YN2-0X-III-Teleconverter-Extender/dp/B00OT0VCP4/ref=sr_1_1_sspa? I own one of their speedlights and it is well built and does a great job with my Canon EOS Mk III and IV bodies. Thank you.