Some of Canon's DSLR bodies can focus down to F/8. Not all of them can only certain cameras can. The Rebel T7 is NOT one of those cameras. Only the EOS 1D series, EOS 5D Mark III, EOS 5D Mark IV, EOS 5DS, EOS 5DS R, EOS 6D Mark II, EOS 7D Mark II, EOS 77D, EOS 80D, EOS 90D, EOS Rebel T8i, EOS-1V (film camera) & EOS-3 (film camera) can AF at F/8.
I am not aware of any extension tubes for the RF mount, either. If Canon has not released a set, then the only third party brand I would trust is Vello. Don’t hold your breathe, though. I would not expect to see them until Canon releases RF mount bodies with more entry level price points.
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