Sigma 300-800 EX DG autofocus issue on Canon R5

Just briefly sharing my experience with this lens on the R5 with Canon EF adapter. ONLY at 800mm (which is a primary reason to get the lens), the autofocus locks at infinity when hunting for focus, and won't budge from there with any AF-buttons until you turn the manual focus ring back. Then it will work until it needs to hunt, reaches infinity, and locks again.

Just curious, does anyone else have third party lenses that do this adapted to Canon RF mount?

I've seen this. I noticed there wasn't good electrical contact between the lens->adapter->camera. I found that if I turned the lens either left or right, I don't remember which, it re-established the connection between the lens-adapter-camera and the problem went away. It was probable counter-clockwise if you're looking through the viewfinder since that's how all three pieces lock together.

This was with an R5, Canon EF-RF adapter but I don't remember which lens but it was a Sigma zoom.

dangermoney wrote in post #19546043
I've seen this. I noticed there wasn't good electrical contact between the lens->adapter->camera. I found that if I turned the lens either left or right, I don't remember which, it re-established the connection between the lens-adapter-camera and the problem went away. It was probable counter-clockwise if you're looking through the viewfinder since that's how all three pieces lock together.

This was with an R5, Canon EF-RF adapter but I don't remember which lens but it was a Sigma zoom.

Interesting - I'll have to try this and see if I can get it to work. Strange that it only happens at 800mm though. Zooming out even slightly, the focus starts working again.