R6 Issues with autofocus with long lenses

[TaraLerch]

Recently upgraded to R6 and works great with my Canon 70-200 and Tamron 18-400 lenses.  However, I recently purchased the Canon RF 800mm and I am having a heck of time with the autofocus.  It works sometimes but other times it freezes.  It appears to be focusing and then gets completely out of focus and gets stuck there and will not focus - not matter how long I keep the back button focus button pressed.  I have to turn the camera off and back on again and sometimes that doesn't even work.  I also rented the Tamron 150-600 and I have the same issue with that lens - not as often but it still occurs frequently.  With the zoom len, I can zoom out and focus and then zoom back in again to resolve but that takes time and shots are missed in the meantime.  I thought perhaps it was some settings I made but I recently reset all setting to default settings and I am still experiencing this issue.  I have completed the latest R6 firmware update and this did not help.  Do others experience this?  I've posted in another group and had at least one person with this camera lens combo that does not have the issue.  I'm stumped - perhaps I have a defective R6?

[Waddizzle]

This setting should be in all of the R bodies. This screenshot is for the EOS R, so your setting may be on a different menu page within the same menu group.

[nianchen]

Thanks! So you think I should turn it off?

[Tronhard]

Thanks! So you think I should turn it off?

Definitely think it's worth a try.  The phenomenon it identifies is very similar to what you seem to be experiencing. 🙂