My repertoire of Sigma lenses to go with my R5 include a 10-20/4-5.6, a 28-70/2.8, a 150-500/5-6.3 and my best is a 120-300/2.8. All lenses are EF and work flawlessly with RF converter. I bought the 120-300 (A$3000 )at the same time as a Canon 400/F4L (10,000) and I could barely tell the difference. I also have a Canon 2x III teleconverter that also has no issue with my Sigma's. The cost of all those lenses was half what it cost for the 400/F4. I would highly recommend Sigma for their clarity and compatibility to the R5 and the RF adapter. Don't get me wrong, I do have Canon lenses, (MPE-65, 70-200/F4L I, 100/2.8 macro) and they are awesome,( but $$$$$ ). My Sigma's do what I need and some of the images I have posted here are with those lenses. I recommend them as alternatives to Canon.
