Hi, I just bumped into this forum looking for solution about my Sigma lens with my Canon R5c.
So, I have a moderately recent 17-50 2.8 that works correctly in video mode, and another two lenses (70-300 , 8-16) that works in photo mode, but not in video mode.
I have sent both lenses to Sigma, they updated my firmware, but unfortunately they both don't AF in video mode.
I have also contacted Canon, and they said they contacted the firmware department to see if they will create some compatibility in the next update.
I think, the compatibility factor comes from how old is the lens, but there is no way to be sure.
There is a list from Sigma of the compatible lenses (for R series), and they say the 8-16 is compatible, but in reality, it isn't.
So yes, just wanted to share my experience in case someone has the same lenses and thinking about upgrade.
