Since this seems to have become a thing that some folks believe that legacy Canon native EF lenses are not working with the Canon EOS R5, I thought I would go and shoot with a couple of my own to deal with the issue that this might be systemic, rather that specific to the users or their gear. The complaints I have encountered so far seem to suggest that these lenses would not focus with the R5.
For this exercise I went to the local zoo on an overcast, occasionally showery day. I took my EOS R5 with the EF 70-300L f/4.5-6L lens, released in 2010. I deliberately tried to find images that would challenge the focusing system, so I chose a range of conditions including moving animals, very small subjects in very dim conditions, some at minimum focusing distance and others using maximum magnification with the 1.6 crop factor enabled. So, these are not necessarily works of art, but they do challenge the focusing system.
Overall, I had very few that were not keepers: of the 96 that I shot, 8 were not good - 2 were me shooting the ground as I handled the camera awkwardly, and one was absolutely massively out of focus - user error. The other 5 were very slow shutter speed resulting in movement by the subject or me being knocked by the many visitors.
All shots were taken hand-held, in available light, shot in both RAW and JPG. Since the files are enormous relative to the maximum allowed here, I have downsized them in Photoshop and allowed some basic exposure and colour correction, none of which should impact on whether the focus was correct.
So here goes:
Looking at the front eye and the area around it, it looks like focus was bang on to me
This series was shot through a fairly grubby thick glass window. I was particularly interested to see if the camera would latch onto the bright plant, but no...
This baby orang was right by an extremely grubby window, at minimum focusing distance and very low light, and I was in a bit of a melee to get a photo as it was very cute and attracted a lot of attention.
These two were taken of an Auckland Green (AKA Elegant) Gecko, in EXTREMELY dim overhead light, at min focusing distance, also through glass. This animal is barely 125mm (5" long) from nose to end of tail. Initially I had to use manual focus to gauge the minimum focus distance (since there was a pane of glass in the way), but once I went back enough, despite the extremely dim conditions, the focus locked on the eyes.
This leopard was shot through thick glass.
Please click on the files to view in them in full size, as uploaded.
I would welcome your conclusions at to the effectiveness of this lens with the R5 body.
